I have the same situation, except SD16 lives with us 50/50 and is unvaccinated because she had a deadly reaction that almost killed her as a small child -not due to anti-vaxxers. I have polled the internet and done a lot of research, and the long and short seems to be that exceptional hand washing and watching for symptoms in the first weeks is sufficient, because baby will soon be vaccinated themselves.

So for the first two months, if you want to keep SD from meeting baby, you’ll have to get dad to give up visitation for that times. I would focus on dad, because other people’s opinions frankly don’t matter here and he’s the one that ultimately decides whether or not SD is in the house.

The alternative would be for you to leave for two months, which I’m guessing you won’t want to do with a new baby.

As far as legal things, I don’t know that there’s much recourse except a notarized agreement that modified the agreement for two months, which requires BM to play nice, too.