My kitten pee's and poo's everywhere but the litter box, HELP!!!!?

The cat is fixed, so that can't be the reason (she is not marking her territory). Well, I adopted this kitten along w/ another girl kitten from the same litter, and these 2 kittens look exactly alike too. The other kitten uses the litter box fine tho. So, what's causing this and what can I do about it? My bf wants to return the kitten, but that's such a shame, and heartbreaking especially since that will seperate the 2 twin kittens.