OP two points you made:
Anonymous wrote:Out of the 5 sibs my husband is the only one with a stay at home spouse and long story short I felt obligated to take the kids until they found other arrangements.... My husband agrees with me and is pissed they don't have a plan B but he is really concerned about his mom and doesn't want to rock the boat right now, as treatment is going well but there have been some bumps along the way and the future is still unknown. He basically wants me to suck it up in the name of his mom.
Your husband is one of 5 kids, but you are the only SAHM in the group, so that makes you point person when babysitting help of this type is needed. Or so you think. The other siblings could, however, be tapped to help in other ways -- such as helping this family afford to pay for childcare until they can afford it themselves.
Now, I know that sounds absolutely ridiculous... why would any siblings chip in for daycare for this family?? -- but it is exactly the same type of help as you are giving, and have been giving for 4 months. You need to stop, and let your SIL know that you can't help out with free childcare anymore, but that she could ask the other siblings for help.
It sounds like one sister has already offered to take 2 days a month off to help out, so let the other 3 siblings know that they are needed to help out as well. Your husband will very likely see that they are NOT willing to help out, and might come around to the fact that you have done FAR more than your share of helping in family times. His sister needs to figure something different out.