CD asked whether or not if was ethical of Mike to accept if Brian and Al wished we wouldn't.
Yes, and that's a perfectly straightforward, reasonable question, and one which has nothing to do with whether Mike is legally/contractually allowed to play the show.
If someone is saying there's a moral or ethical issue, then that is a 100% subjective, opinion-based discussion. Answering that subjective, moral/ethical question with "it's Mike's choice whether to do the gig" is a rather Cam-esque dodge in my opinion.
Or it's my opinion.
If your opinion can include answering a different question than the one that was asked, then yes.
No, my honest opinion is that, if Brian or Al morally object to Mike's playing on 1/20, it doesn't matter because Mike controls the brand name.
That's my answer to CD's question.
So you have no problem with the idea of a bandmate - and relative - believing their fellow bandmate/relative's moral objections are of zero concern to them, and that it's perfectly ok to do absolutely whatever they want just "because they can"?
If Brian really objects to this, he can get on the phone and contact his cousin. But unless that happens, I see no issue with Mike taking the gig