I'm aware that my reply is a couple of months late.. but I sure hope that your situation has improved in the mean time. I was just reading this post, and I wondered: maybe you and your sister could write a letter to your mom? You can put your feelings in it, tell her everything you want her to know, then you can put the letter away for a while, you can read it over and over again, choose the right words, and someday when you feel the time is right send it to your mom. And do not feel guilty about it
From what I have read here, you and your sister are the hurt ones in this story, so it is your mother who should feel guilty. The advantage of writing a letter is that you have the time to structure your thoughts, while your mom can't interrupt you and/or manipulate you. And sometimes I find it easier to send a letter than to confront someone face to face. I always like to think that when people receive a letter with criticism, they might reject it at first, but chances are that they will read it again after a while and start think about it. And even if you and your sister choose to not send the letter, at least you have vented your feelings. It helps to look at the situation from a distance.
And maybe it is needless to say, but don't let your sister forget her great qualities even when your mom fails to see them! I can imagine that if I had been in your sisters (or your own) shoes, being criticized and not accepted, I would feel worthless and would actually start thinking I am the problem. Don't let that happen, because it would be a completely irrational thought.