To not say something when someone is being rude

[DuchyCazalet]

DB can be quite rude to my DF, along the lines of being dismissive of questions or a general attitude of sneering. He's mid 40s so not a question of immaturity. I've also seen the same with SIL to my MIL. It can be quite uncomfortable to be present when this rudeness takes place. Neither DF nor MIL comment on the rudeness but it is very obvious and must be painful. I have to bite my tongue but don't say anything as the rudeness is not against me and I also don't see either of them that often (live in different cities) so it's just not worth the fallout.
Just feel that I'm part of the problem by not saying something though. AIBU?

Yanbu, call them out on it. How dare they. They aren't being challenged so they think they can get away with it.

Depends 🤷🏻‍♀️

You could call me a bit rude how I answer my mum sometimes but I know her intentions behind what she says and you don't. And I'm not going to be quiet around racism or wtv!!

I did this to my dad when he was really dismissive and rude to my grandmother (his mil). But I did it later and when we were alone as as soon as someone feels shame - they can often attack rather than reflect.

I said something alone the lines of .. are you aware that when you said xxx it comes across as patronising?

Are you not particularly close? I'd stay out of it if I wasn't close, but would say something if I was.

I've had a similar experience with my FIL being rude to my MIL about a meal she'd made, I was a guest, and didn't feel it was my place to say anything. It was very uncomfortable and I did go a bit over the top with praise for the food, because it was nice and she'd tried something different. DH has improved since then and challenges FIL's bad behaviour... there is some dysfunction there (putting it mildly).