As others have mentioned, you're NOT in any way the owner of a home, unless your name is on the deed, or documents are drawn up to include your name on the house. Until that happens, no matter how much you pay, it's not your home. I think a bit of what Nervy Girl and birdsfreakmeout describe is what is happening here. This woman probably appreciates her privacy, is an older woman, and was finally looking forward to a life of independence and being an empty nester, but now her son comes back home, with his kids from a prior relationship, a new wife, and a baby on the way, to force them upon her and interrupt her peace and bliss, so now she feels she's not done raising kids and no longer entitled to the reward of a peaceful retirement and a quiet home.
Put yourself in her shoes. You have raised your son, and now that you want to live your life alone and quietly, all these kids are shoved back into your home, along with a new spouse and a child, and you're made to wait on them hand and foot and love them, no matter how they behave toward you. Wouldn't you be resentful? I know I would. You assume when your kids are grown up, married, and moved out, that they will build their own life, not return and be a burden, and obviously, you ARE dependent on her in some way (financially, child care, or both), or you two would never have agreed to this arrangement in the first place, so in some way, you ARE a burden to her -- whether it's having to do more housework because the kids don't clean up after themselves, or acting like a maid, doing your laundry, cooking for you, and cleaning for you, and without any appreciation shown for all she is doing.
It's one thing to temporarily move in, like say your husband loses his job, you're in the middle of finding a new place to live but your current lease is up, or you're having a dangerous pregnancy that requires monitoring and your husband must work, it's another to move in permanently, with no move-out date in sight, and then to act like you own the home and this woman must kiss the ground you walk on and be your personal maid because of some financial contributions. The fact you have this sense of entitlement about this woman's home makes it clear you don't respect her and feel that she owes you everything. You reap what you sow. If you want her to show kindness and appreciation toward you all, then be prepared to do that for her first. Thumbing your nose at the home's owner is not the way to do that. You talk to her, show affection, gratitude, concern, and do nice things for her to earn her love.
Think about it, you would probably be praying every night that your son and his spouse move out if she is constantly making claims that the house is hers and that without her and your son, you wouldn't be able to survive and rub in the fact that you need them to survive. Make an effort to help around the house and change your attitude towards this woman, and make sure her grandchildren do that as well. It's the least they can do, considering that is their grandmother and they are staying in her house, so they need to clean up after themselves.
Remember: she CAN kick you all out in a heartbeat, so think about that next time you want to get in her face about the bills you pay, because you may end up having to suck it up and find your own place to live. You have two options: show some humility and appreciation while keeping your mouth shut because you are dependent on her, or if you feel that you cannot do so, then talk to your husband about moving in with other people (how about your own mother?), or renting your own place because you are unhappy and uncomfortable in his mother's home and there is no other solution to this situation.