What a refreshingly cool post, good on ya mate
My take.
Stressing about it won't do you or your health situation any good at all. I think you've done a good thing by posting & getting it off your chest & i am hoping this thread will be enough to destress you enough to be cool.
I don't think you should approach her or her mum about it & just rest easy. You Have to be the nice bloke she fell in love with not a stressy over paranoid worrier, I mean that in the Nicest possible way mate i am not having a go at all.
Busy yourself around the house, Make it nice for her & the kids to come back to. Do the shopping & plan a couple of nice home cooked meals, Be Happy to see her. Shelve those worrys until something bad actually happens & like i say be the man she fell in love with.
Good Luck