My brother & sister-in-law were particularly protective of their baby daughter (first child), to the extent that they would barely let anybody else even hold her. They've gradually relaxed, and now that she's 1 year old have gone so far as to allow our parents to babysit Abby (at our parents house) for a couple of hours in the afternoon each week, giving them some much needed alone time.
All was going well until a few weeks ago, when they asked our parents to have Abby one evening while they went to a friends house for a group gathering. They said they would pick Abby up from our parents house at 9pm. Our parents had Abby ready to go at 9pm, already for bed so they could take her straight home to sleep. At 9:20 my sister-in-law called to say they were slightly late leaving but would be there in 20 minutes. Unfortunately, by now Abby was getting upset - she was tired and had been away from her parents for longer than ever before. They eventually arrived at 10:30pm, by which time Abby had been screaming (despite my parents best efforts to settle her) for over an hour. My Dad was absolutely furious - not because they were late, but because Abby had been so upset and he didn't think it was fair on her. My brother & SIL couldn't understand why Dad was angry, and basically have indicated that they won't ask him to babysit again if he's going to react that way. They haven't spoken to my parents since, nor have my parents reached out to them.
My question is this: as an uninvolved family member, should my role be switzerland? I'm not going to stick my nose in, but I'm wondering if I should say anything when the topic arises, which it will. If anything I'd consider a gentle nudge towards understanding and reconciliation. My Dad and my brother can both be quite stubborn and arrogant. So something that's quite minor could turn into a lengthy and unnecessary estrangement. It's silly because
neither of them are actually upset - they're all basically over it - but neither will be the first to talk about it, they'll just assume the other one is still mad and avoid each other.

I know from past experience that as soon as they do talk to each other, everything will be fine again. Thoughts?