I use a product called Nature's Miracle, which is available at pet supply stores. It's worth every penny. When there's a carpet problem, I pour it on so it soaks through both the carpet and the pad. It has a fragrance of its own that isn't all that pleasant but, when it dries, both its own smell and the cat smell should be gone. If the cat smell is still there, I know I've missed a spot. So I have a little black light that tells me where to aim (a black light is a very good thing to have if you have a cat - especially a male cat).
I'm not saying you might not have to replace the carpet, if Mr. Kitty has done too much damage - but this is well worth trying first.
You can try it on the coats and on the bookbags as well. Can't hurt! I've used it on books and upholstered furniture.
If you have any NM left over, you can give it to your sister. Male cats, bless 'em, are territorial - even when they're fixed, although they're not quite so bad at it then. You want to remind your sister to change his litter box frequently so he always has a clean place to answer the call of Nature and won't look for another location.
P.S. I just read Bobbi's post. I've had her experience! When we moved into this house decades ago, there was a spot we just couldn't keep our cats and dogs away from. When we pulled up the carpeting, we found a spot in the hardwood made by the previous owners' pets. I remember using two applications of full-strength Clorox to get the smell out! Then we covered that place in the dining room with an area rug until we had the floors redone later on.