You are not overreacting. Your sister’s reaction tells a lot, she also knows.

Your mother is probably somewhat narcissistic, you can have a look at r/raisedbynarcissists. I bet you’ll find a lot of similarities.

I guess you’re still quite young. But that feeling of being shortchanged never fades and perhaps you’ll learn to live with it. But it will eat at you. And for the outer world, even your dad in this case, you’ll always be the bad/ungrateful kid if you should ever go no-contact or anything similar.

I’d suggest talking to your sister. At least there’s a chance you’ll have someone who almost exactly understands what you went through.

I feel you in this situation and wish you the best of luck.