This has become quite the complicated and fascinating post! It’s very interesting to see how opinions have evolved. Everyone here has valid pros and cons, but I sense, @conmama that there is a real risk here of long term damage to the sons’ relationships.
I do not agree with some other posters here. We are always going to be parents. I’m not going to withhold sound advice from my grown children just because they are adults. If I saw something occurring between my kids that might possibly cause a life long rift, I would speak up to BOTH kids.
It’s possible they won’t appreciate my meddling. But I would argue that it is worth the risk. It seems clear that older brother is taking some sort of advantage and younger brother already resents it.
I’ll wager everyone here knows someone or even IS someone who has seen family relationships ruined over what perhaps didn’t seem to be a huge deal at the time. Family is for life. I know a lot of people who really regret family misunderstandings and who wish they could go back and fix things.
My stepbrother once caused a big family argument. Eventually, my dad stepped in and basically told all four of us to stop behaving like toddlers. It was surprising that he intervened (sadly, it didn’t work out), but I admired my dad for doing it. He was right to have stepped in, even if it didn’t go as he would have liked. We still respected him.
ETA: Perhaps a good approach could be: “I’ve spoken to to both of you and I understand your miscommunication, but you would both benefit from discussing this further before it causes long term damage.”