I hear ya and I feel for ya but sometimes, unfortunately, that's just how life is. It sucks and I don't agree with it, but as you've probably figured out, this is how it's always going to be, unless something drastic happens to change it. Question now is, what are you going to do about it? Life is what you make of it and only you can make or break your own happiness. It's very unfortunate that your family treats you that way, but by now you should be able to recognize it for what it is and try not to take it too personally. It's your choice to allow it to hurt you and frustrate you and make you miserable or it's your choice to realize your sister needs an butt whooping she's never going to get and shrug it off to another one of her antics.
You should be very proud of yourself that you are able to accomplish things on your own and of everything that you already have accomplished. Be proud of that fact. Piss on anyone who tells you that you can't do something or puts you down. Fact is, as you've already proven, you can do anything you set your mind to. If you want to go back to school, do it. Get financial aid or whatever. You can do it. Think of yourself and your hubby and future children and put them and yourself first.
Frankly, I'm quite proud of you from what you've said and I think you are doing a grand job of making it under the circumstances. Be proud.
p.s. I think you and hubby need a home of your own.
Good luck honey