No way around it -- this is a mess!

I do think, however, that your SIL probably knows more than you think. It is her mother, after all. Assuming that SIL and your DH were raised by her, I think they must know of her limitations.

You don't give a lot of details about relationships, but all things being equal I think it would probably be a mistake to step in between mother and daughter, unless it was necessary to protect the child from danger. (In other words, if the behavior was abusive to the point that you would step in for any child.) Sounds like these 2 have a pretty enmeshed relationship -- whose mom does so much work for a healthy adult woman? -- and it's unlikely they will appreciate or even acknowledge the perspective of an outsider.

It's too bad DH has so far said he won't do anything -- whatever could be said would be so much better coming from him.

At most I think you could safely express concern for your MIL in these situations and offer help. If you are present when these things are happening, could you say something like, "Boy you've had a long day. I'll watch the kid for a few minutes and you sit down." Or even something like, "You deserve a vacation from this!" Or, "I don't know how you do it."

You might not get an immediate change, but knowing that someone cares about her might make MIL feel less overwhelmed, which could only help. And in time it might lead her to say to herself, "Hey, I am overwhelmed, this is too much for me. I need to limit the number of hours I do this."

And there's always the classic technique of praising anything you see going well so that she'll do it again. Like, "Good for you! You should take a break from babysitting more often!"

Shaun
www.homeschoolblogger.com/shaunms