Miss Manners would recommend that you stop giving the gifts. If the niece doesn’t like them enough to say thank you, then it’s a bit pointless. Think of it as teaching your niece a very valuable lesson for life; you can always resume the gifts once your sister gets the idea. This is not really to the point, but it’s the best I can do for the moment. (Go to the library and get one of the books.)
I myself would say that $100 is an awful lot for a present. For an only niece with well-off parents, I’d spend more like $25, unless it’s a new baby gift. Gifts within families should be understood to be tailored to incomes.
I can answer this from the other side. I have gotten nothing (or small pieces of crap) from my brother and his family for birthdays/Christmas since he got married about 15 years ago. He has four children now, so I’ve deliberately refrained from doing the $math because just thinking about it makes me cranky. I sent gifts for Christmas already, but I think this will be the last year I will do so. If I got thanked occasionally I might have a better attitude about it.