I have a certain relative like this. My husband's twin brother actually. He has been very inappropriate many times and has major issues. I don't associate with him. It has made me sort of the big "bi___" of the family, because back in the day I would ignore all his emails despite his begging me to email him. I'm not FB friends with ANYONE on that side of the family, because then I would need to friend him-or it would be awkward not to...He's getting remarried at the end of May, my husband is taking the kids and I'm not going, I told him "no leaving any kids alone with unlce X" and got in a big fight with the hubs because he was offended I don't trust his brother with the kids (dont' worry, I won the battle and the brother will only be around for his vows with lot of people and gone the rest of the time so he won't have time to lurk around the kids-plus he's only a perv with "ladies" as far as I know, not kids).....I've bowed out of many family events when I didnt' want to deal with him making ME the anti social "difficult" wife....you get the drift. There is no way in hell anyone in that family especially my MIL would EVER admit anything was wrong with him. It was really awful at first because I wanted my husband to not like him too! But of course minutes after I told him what he did, he was back to normal like nothing was wrong with is brother. :( Thank god he lives far away.
Anyway, yours is harder, because he's around. You're going to have to put your foot down to your husband that you don't want to be alone with the guy EVER and you don't want to be around him much, so be prepared for your bowing out of most things if the guy is there, and you don't want him at the house. Tell your fiance why, but don't expect him to get it. It will be all up to you to enforce. I f you want to stay in the kid's lives, you'll have to go out on a limb and handle the guy.
Once the BIL came up to me when pregnant and wanted to rub my belly (after several other yucky molestations) and I said, "NOPE, YUCK, no WAY I hate people touching my belly." Right in front of everyone. Once again, I was being "abrasive" but oh well. He was a close talker and I said, "Sorry, I need space when people talk to me, please stand back, I've always been that way, sorry, I don't like to be touched, nothing personal." I KNOW it's so hard to have to say those things, but otherwise he'll have control.