How can we fairly and legally allow my mother to contribute monies on a monthly basis to our household?

The best way to handle this is see an elder attorney to draw up a contract which is legal binding and totally protects you from any interference from your siblings, but if you don't want to do that - this is what I would do. Check around and see what Assisted Living or Senior Living would cost so you can get an idea what she would pay if she lived somewhere else. Then draw up a contract stating how much she will pay you a month, have her sign it and have it notarized. Place this with her will - if you have it (if she doesn't have one - I'd get one immediately). And don't feel bad about accepting money from your mom. It's HER money for her care or however she wants to use it - not your siblings' future inheritence. My mother-in-law lived with us, and outsiders have NO IDEA how your utilities, food and everything else go up having an extra person in the house. Plus, it will help her and you feel better about your family being her caretaker. It's easy to live on the outside and say a lot of things, but they don't have a clue what being a caretaker is and how many hours, family time and loss of sanity sometimes being a caretaker takes. So don't feel bad about taking it and tell your siblings to take a hike - or ask them when they'd like mom to move in with them? Good luck!!

Please, please do allow your mother the dignity of paying her own way. She is not a charity case, she and your Dad planned well and has some funds for her old age. Respect that.

It is NOT your responsibility to conserve your mother's funds for your siblings to inherit later. Obviously you don't want to rip your mother off, but if your sibs think that paying you is infringing on their inheritance, send them copies of what it would cost for mother to live elsewhere. As vw9729 says, it is your mother's money, not your siblings inheritance.

You do need to make this official and make it clear that mother approves of this arrangement. This is to protect you from groundless accusations by siblings and also make clear that this is not "gift" money in case mother ever runs out of funds and needs to apply for Medicaid. If you've used an elder law attorney for things like POA and Medical POA, etc., go back to that firm for a simple contract. If those other documents are not in place, it would be a good time to have it all taken care of.