I have an aging MIL myself, and she is in somewhat poor health.
I would back off and let your SIL and husband handle it. And by "handle it" I would let my husband know that I will take no part in cleaning up a mess created by failing to plan. Of course I would say this in a nice way. If he complains to me down the road (or my SIL does) I would say "well, I know you guys will figure it out."
In other words your SIL has made it fairly clear that your very practical help is not welcome. So it's time to back off. Whew, check it off your list.
It's too bad for them, because sometimes it takes someone a bit more "outside" the situation to think practically, and get things done.
I've gotten overly invested in my MIL's situation (and I love her alot) and regretted it at times.
Good luck.