I think your husband needs to stand up for you and his mom in this scenario. It will still be ugly, but much better coming from him.
This is the key. OP, You're a good person for doing all this so far. And I understand your husband doesn't want his Mom taking on childcare again, so that needs to be a part of the conversation. Your BIL and SIL need to figure it out. It stinks, but taking care of 5 kids is too much. Your husband shodul have a conversation with his brother and tell him, straight out, that you will not be able to continue to do this. Then your husband and ALL his other siblings should resist any attenpts to foist the kids back on MIL, or evebn bring it up with her. It isn;t her problem, and she shouldn't be involved.
I'd guess you are being made to feel like you are doing a nice thing for your sick MIL, not for your SIL. That's incorrect, and you should resent it - hell, I resent it on your behalf. You're doing a favor for a healthy, able-bodied adult with a job and two kids. You have every right to put a time limit on it - my limit woudl have been far shorter than the months you've already done it.