You're right here, SIL really does feel caught in the middle here. She naturally is more like her father, not her mother and gravitates to him more, she and mother clash a lot but things have improved a lot since SIL had DC.
SIL however, would not (and DB wouldn't either, 18 months spent living near them over lockdown was enough for him) move back to live near them to help, unless the worst came to the worst and they really needed them there. It certainly wouldn't happen now.
Every year though on most holidays SIL and DB and SIL's DB and his DW and kids take it on turns to take the DP's on holiday. I did suggest to my DM last year (not to anyone else) that maybe DP's could go on holiday with a private carer by themselves once to see how that went as a trial run as it would take the stress off DB and SIL.
One of the reasons I said this was last year it was SIL's DB and his family's turn to take them on holiday. My SIL and DB and their family (but SIL was pregnant then) said they'd join them for a week and then go on to their own accommodation in a different place same country (France). SIL's DB though on at least 2 occasions went out with the godfather of his DD who was visiting from abroad and got drunk and left his father to call out for help getting to bed (he can't do this by himself), the mother couldn't help for some reason (maybe it was the way the accommodation was set out) and my DB had to go and see to him. When SIL's DB returned they either had a huge row the same night or the next day which DNephew overheard.
But also, SIL and DB have told us that SIL's DB and his DW have very different parenting styles (stricter) than they do, which don't align with theirs. But they said they probably wouldn't go on holiday again with SIL's DB and his family as they could see there would be huge rows again.
As is the case in so many of these scenarios, I really don't want to get involved but if I see DM, she naturally confides in me about DB as she's his mum. He also has his own stresses and health issues. But generally I try to stay out of this, and I never comment because I know it's not my business and DM also tries not to comment either. She did send DB's MIL a polite email about them complaining to DB about his job though and said he is a kind and caring husband and father who does a lot which he does. How many son in law's would look after their disabled father in law? My DB does this a lot and doesn't mind doing it. He's even done this alone and I've travelled to their house with him when the FIL has had to be left alone but needs caring for overnight.