Remove the carpet.

If you can remove the carpet, do it, but make sure the landlord is paying for it. If he or she isn't willing to remove it, have the landlord pay for the deepest cleaning of the carpet money can buy.

If she's that allergic, she should consider asking the landlord to split the cost of replacing the carpet. A hallway and room may cost around $500 if you shop for budget carpet. You may also find a great deal at a remnant warehouse that offers a $99 installation special. I found a Wool berber carpet remnant for $12/yard (normally $50/yd) and did my entire house for a $99 install special.

PS -- You can also ask to have your deposit reduced for by the amount of your part of the carpet investment. Then add a clause to the lease that says you agree to pay for a professional cleaning service, not to exceed the cost of 1/2 the carpet price, when you leave. You've deferred the cost to your move-out, rather than having to pay now, and funded new carpeting with the reduction in your deposit. Sweet deal -- worth asking for. The worst they can say is "no."

PS.S. -- professional house AND carpet cleaning service...

Sprinkling baking soda throughout the room for a while then vacuuming seems to be a very cheap, decent way of getting odors and what not out of the carpet.

if your sister and/or the landlord aren't set on having carpet in those areas, just have it taken out. if there is hardwood in the rest of the apartment i'm sure there are decent hardwood floors under the carpet. it would probably cost less to do a refinish on them then to install new carpet.

I deal professionally with allergen abatement. The bad news is that pet dander will persist for six to nine months in a home after the pet leaves, even with professional cleaning. If allergies are severe and it's possible to live elsewhere, that may be the better choice. If not, definitely tear out carpet, and ideally replace with non-carpet flooring. Pet dander is small and airborne, so borrowing a HEPA air purifier and running that for a few weeks may help. Wash any drapes or other linens in hot water. And give the apartment a thorough scrubbing and wet mopping - pet dander will stick to walls, cupboards, baseboards etc. If the allergy sufferer has to do the cleaning, consider taking some allergy medication preventively (anti-histamines, steroid nasal sprays) and wearing a protective mask. Ideally, the allergen abatement can be done by a non-allergy sufferer.

If it's in the carpet, it's likely elsewhere too:
HVAC ductwork, Behind/Beneath appliances, etc.

Personally, I'd look elsewhere for a livingspace - There are plenty of non-pet apartments out there.

That immediate telltale tickle is a red flag that she's highly allergic to the allergens still there. She shouldn't move in unless first all carpets are removed, all paintable surfaces are painted, the entire apartment is cleaned from top to bottom, and afterward she visits it and doesn't develop a telltale tickle.
Even that wouldn't guarantee that she could stay there.
Allergies that are tolerable while standing can make it impossible to breathe while lying down, much less sleeping, making a home unlivable. Allergy medicines and immunization therapy help only so much. Sorry.

Shouldn't the landlord be removing the previous tenant's carpeting anyhow? If not, if I were her I would just pay to have new carpet put down. Regardless of the allergies, prior used carpet is just nasty.

Keep looking...

@cherrybomb - doing that every time you had a new tenant would be costly. I know most apartments, whether owned by individuals or by companies, replace carpets every few years, and will skip replacing it entirely if it's been kept in good condition.

I have the same problem. I would suggest fresh coats of paint and new carpets if possible. If the landlord is not willing, then see if you can get the walls steam cleaned and a deep cleaning on carpets. Personally, I would consider looking for another living space, but I can understand if it is a great find....

Look elsewhere. Even if the carpet was removed, it will be forever until the dander breaks down.

I second cherrybomb, I thought landlords had to replace the carpeting with each new tennant (which is why they usually have such bad, cheap carpeting in some rentals).

I'm highly allergic to cats, and one good steam-cleaning on the carpet worked for me actually.

Get rid of the carpet.

I lived in a number of apartments with dirty, old carpets in Florida. I think that rental dirty, old carpet isn't illegal here, although code varies by time and location, and a lot of illegal stuff happens to tenants despite local laws.

Another vote for removing the carpet...

I recently had a potential landlord suggest that he would remove carpet in his unit, if my husband and I were willing to sign a two-year lease. Is your sister willing to sign a two-year lease? If so, begin negotiations with the landlord. The worst they can say is no and she will have to keep looking.

I had a landlord who let me take up the carpet because there was hardwood underneath, and I was supplying free labor. You might also convince him to pay for laminate (if there's not hardwood underneath) if you put it in. It's suuuper easy and quick to do. My allergist told me there's almost no way to get cat dander out of carpet, dog dander is easier but apparently cat dander has little spikes on it. So what kind of animal are you allergic to/lived there?

I am allergic, and my doctor advised me to go all hardwoods. Every time I see him, he makes sure I haven't added a rug. He wants NO textiles on the floor at all. When you have carpet, every step you take sends the allergens airborne again.

It made a huge difference for me. I would advise your sister to find an apartment with all hardwood floors. In the long run, she will breathe easier.

Allergies often are worsened by exposure to the allergens. That means that even if she's OK there at first, over time the allergic reactions may become intolerable. I wish her luck in finding or making a hypoallergenic home. Sadly, there's no such thing as a nonallergenic one.

When we moved into our 5 bedroom, 3 story rowhome rental in November, we rented a carpet cleaner (Rug Dr.) from the local supermarket and spent the first three days cleaning and scrubbing every square inch of the place. We opened all the windows gave everything a thorough wipe down with bleach and the like. I'm not severely allergic to pet dander, but the cleaning definitely made an impact. We also painted all the walls (with zero VOC paints). I think it's pretty unlikely that the landlord will remove and replace the carpets, so if the place is worth it, then you'll do what you have to do!

ihateketchup, I did similarly after moving into this home, except that I removed and discarded all the carpet (mortgaged, not rented). Fortunately, there was terrazzo under the carpet, so I just left it bare. All considered, this was the best available home that I could afford, so it was worth it. The work took a long time, and made me very rashy and wheezy. It would greatly help the initial poster's allergic sister if some one else were to do it, as another poster mentioned. The work would be much easier if done before the apartment is occupied, but finances rarely permit that.

Oh wow, I had the "pleasure" of dealing with this situation at my last two apartments, both of which I was sharing with severely cat-allergic people (I'm not allergic myself). My latter apartment had a cat in it that we (and the landlord) didn't find out about until four people had terrible allergic reactions on the day we moved in (and we found cat toys and kitty litter under the washing machine). Ripping out a carpet might work, but if that's not an option, try shopping around online for a denaturing spray. I used denaturing spray (Allersearch is the brand name I used, but I think there are other kinds available) in both places with excellent results. We actually convinced our management company to pay for the spray at the latter place. It was about $40 (shipping not included) for two bottles, which was enough for our 1200 sq. ft. place. I also invested in a motorized sprayer (worth it to avoid carpal tunnel syndrome). The carpets had been deep cleaned and all the walls were painted right before we moved in to both places, and that was absolutely no help. I have no idea about how "green" the product is, or really anything about the chemistry behind it at all, but it's saved my friends and family not once but twice.

If you can't remove the carpet, keep looking. The landlord sounds willing to work with you so try to meet them in the middle somewhere.

I find it interesting and appropriate that this comes on the heels of the DC AT post where the author wants advice on keeping cats (caring for the carpets, removing dander, etc.) in an apartment she isn't supposed to have cats.

@mandybles I'm interested to know more about the technique to apply. Did you just spray the carpet wait 3 hours and vacuum as instructions state? my best friend is moving in with me and it has been only 3 weeks since the cats have left