While I agree with your take, it doesn't justify the pure selfishness of what they're doing. We've been purposely avoiding situations where we'd expose or be exposed by following the guidelines but they seemingly feel they're better than that. There's a lot more history here with her but, that's a totally different series of stories. Suffice to say that we really don't spend any time together.
I agree with your premise in that I would like to see restaurants open at 25-50% capacity with the necessary precautions in place to minimize transmission. I think that's a much better way to control it rather than having people having private gatherings with no precautions, which is shown to be a much larger spreader problem than restaurants. If they could socially gather in controlled environments, they'd be less likely to gather like they do. At least, that's my thinking on it.
As for my SIL, their justification for this is just bullshit. Add to it that she's bragging to people about getting their shots, even though it's obvious they're cheating. That's just dumb. She should just shut up.