Annoyance...

[Rawr]

So it turned out that my plans to see my sister and to go see him are on the same weekend. No problem, he's told me he wanted to go meet my sister and so on so I asked her if I could bring him... Well, there's one big problem. My brother in-law has cancer (a rare, slow growing type that I know nothing about) and he is going to be starting chemo sometime in the near future, might be a few months away... and my partner I'm suppose to bring has not gotten vaccinated. Now, despite anyone's opinions about COVID, getting vaccinated, understand it is going to be my sister's wishes and I am in total agreement that he should be vaccinated around someone who has cancer, is immunosuppressed, etc. Nor am I going to risk doing something that would jeopardize my sister, her husband or my niece's well being. So, since getting vaccinated is something he'll likely not do, I guess he'll have to stay home that weekend because I don't get to see my sister, or my niece very often or for very long. They live across the state, about 420 miles (675km) away. So I am not going to pass chances to see them.

Hopefully I have given enough context here to ask the question. Am I being selfish if I choose to go visit my sister instead of him?

Not being selfish in the slightest.

Not to get into it, but there has been a lot of arguments around this sort of subject to the point where I think he should be pretty understanding giving the circumstances. 

There'll be more opportunities for him to meet your sister I'm sure and missing out may make him re-think getting the vaccination.