Agree with Alison-- do nothing, continue to work on detaching, make the decision that is the right one for you and the kids.

On the trip-- if you take all the safety and quarantine-related stuff out of the picture, if it were me I would see two possible options for the trip-- one, I would spend the whole time feeling anxious and angry imagining my H with the AP, or two, I'd be able to really relax with my sister, have people around me who love me and care about me, and use the time to truly detach and not give a $hit about what my H may or may not be up to. I don't know where on your journey you are, though it sounds like you're moving well along in the detachment zone. If you can use this opportunity to really invest in yourself, get a breather from him, spend time with your children and your family... that sounds pretty amazing.

If you do decide to go, you really have to be strict with yourself about dropping the rope and not worrying about what may or may not be happening. Throughout the fall when I knew about the existence of the AP, but only that it was someone he had an "emotional connection" with and also he had said they were no longer in contact (which was a lie), I twisted myself in pretzels every time he traveled for work to her city. It was awful and I don't wish that on anyone. I would try to be as honest with yourself as possible about where you are and if you are indeed in a place where you can let it go and use it as a big opportunity to advance your own healing and detachment, or if you aren't there yet. No judgment either way.

(on the safety side-- I would take that all very seriously, though I wouldn't use it as an excuse for yourself to make the decision not to go. (feel free to use it as the reasoning for your H, though.) In my state, the quarantine restrictions are no joke and violators are being arrested. It may not be all that fun of a vacation if you can't leave your sister's house, though I don't know, obviously, what the regulations are there. I'd just do some research on all of that before making the decision and what it would really be like once you got there.)

Me (46) H (42)
M:14 T:18, D9 & D11
4/19 - 12/19: series of escalating BDs
9/20 - present: R and piecing