Are there any legal points so that all four family members should share and keep her?

I think perhaps your ancestry is of a country that expects the oldest son to care for the parents in their old age? If so, that might be what your MIL is thinking. Old ways die hard, but they don't have to carry over to the US.

Since you and your husband had financial responsible for 21 years, and one SIL had care responsibility, it's only fair that the rest of the family step up and accept their own responsibility, especially since your husband is now out of work.

You'll have to be firm on this; make it clear to the noncontributing family members that you can't afford to care for your MIL and they need to become involved, or else contribute financially so she can live in AL or IL. I'm sure this isn't going to be easy though.

This is not a legal matter, at least in the US.

None of you have to have her in your home.

In some states there are laws that apply if the parent is destitute -- children should not allow their parent to be homeless if they can prevent that. Not all states have such laws, and it really doesn't apply in this situation, where the question is about which children have responsibilities.

I would simply not accept an ungrateful and free-loading MIL in my house. Nope, not happening. I would be supportive of my husband, and I would not object to financially helping the woman, but believe me, she would not live in my house.

You cannot enforce family rules about what the other children should do. There are no civil laws about that. But you certainly can determine who lives in your house.

MIL may think it is the responsibility but that does not mean that it actually is, legally speaking. There are filial responsibility laws in some US states, but it doesn't sound like they would apply here. At any rate, you are not obliged to care for her in your home. What financial resources does she actually have? Really morally all the children should contribute to care of destitute parent as far as they reasonably individually able, but probably this is not enforceable. So you and your husband will have to get tough with the sibs for money support so she can go into subsidized senior housing, assisted living, whatever is appropriate. None of you have to have her in your home, nor do you have to provide care if she is able bodied.

Adult children are not legally responsible for their parents care in the US.