Hi,
Long time lurker - MNs have some great insights so I'd really appreciate your advice on this tricky family dynamic.
I've known DH's family (his parents and 4 adult children) for 20 years and have always thought their close bonds were lovely - they're all v. different personalities and scattered around geographically, but they make a big effort to stay in touch. My PIL have always been helpful in practical ways, e.g. if they visit, they'll fix things around the house. They've done this for everyone, but nowadays, DSIL2 is the main beneficiary as she lives the closest (a 90-mile drive away). In recent years, They're at her house a lot, either fixing things or looking after her pets if she's away. She refuses to use kennels or a pet sitter.
Anyway, here's the AIBU question: PIL are in their late 70s and take care of themselves, but some health issues have cropped up and MIL started having severe back pain a couple of months ago. She's having therapy but DSIL1 (who I'm close to) visited recently and said it's pretty bad. MIL is on strong painkillers just to move around. She often can't lie down to sleep. 
DH rings his parents every weekend and was shocked to discover they'd just got back from DSIL2's house on Sunday (a 180-mile round trip) as they'd been checking that a new appliance was properly installed.
MIL's back was bad that day...what a surprise, after 3 hours in the car over the weekend. 
I was appalled, tbh and think DH needs to say something to DSIL2. It could be diplomatic, like we all need to recognize that PILs are getting older and we need to take care of them. My family is v. different, though, the younger generation are expected to do more for the older folks and we don't mind a good row if we disagree on something. DH's family never argue or interfere in each others' lives - that's why they all get on. 