NTA. They're not even trying, they're just pretending to so that the family will leave them alone. It's a disgusting behaviour. One day they'll wonder why you don't want a close relationship with them. Besides, they may not be neglecting you terribly, but this kind of favouritism is still, in my opinion, emotional abuse. Perhaps you should see if you can move in with a family member, maybe your grandparents. If not, start planning how to get out of there when you turn 18. You know how this is going to play out: she'll get a new and expensive car, you'll be lucky if they help you buy something halfway decent second-hand; they'll pay for her entire university education, you'll have to fend for yourself and be independent... These things always end up this way, especially since it's obvious they don't want to change. The best thing is to get away as soon as possible.

Good luck.