Separated, MLC and Infidelity - with kids

[Optimust]

Horrible 4th. Started out ok, went down and played golf with FIL, he and his wife love W but are not happy with her activities, and warned her about finances and our DD’s. Golf went fine, little talk of W and I, just to be careful and protect myself and DD’s. W left on the 3rd about 8 pm and did not return.

Left to go home and get D14 and take both to a friend’s at a lake property, but weather was to aweful, so D14 suggested going to local street/firework festival with a friend of her’s and D8 and me at same event(not with her tho, to embarrassing to be with dad), then abruptly changed her mind. She was at W house just up the street from ours(W not home), and I went up and found OM car in the garage. D14 knew who’s it was. Still didn’t want to go with me and D8. We left and 20 minutes later D14 is TM'ing me, ‘can mom pick us up’, I reply,‘to go where?’, no answer. W then calls and says she is going to pick her up and take her to OM friends place where she and OM are. I ask, then I ask firmer, then I warn, then I plead for her not to do this, because it will mess up D14 badly, she does it anyways. She drops D14 and her friend at my house at 11 pm, and goes to her place with OM. D14 spends night at my house under duress, and when I take D8 to swimming in the morning at 11, she goes to W’s(only 1 block away), and OM is still there!

Then D8 tells me this late last night:
W told D8 that if D8 does not try to get us back together, she will not make her do something she doesn't like to do, forever.

What would be a boundary I could set to stop her from involving my DD's?

I'm trying to get the whole boundaries thing, but not sure what I can do. I do appriciate any help.