Originally Posted by SNeacail
Not knowing any of you personally, I would hazard a guess that at least part of Matt's dislike for Si is deeply connected to the fact that she was a threat to his role within his family and he has no guarantee that she still isn't, only time will tell. In his eyes, the two of you were trying to steal his kids from him, how would you react? Usually when we introduce friends and extended family to our kids, there is an expectation of some distance. I wouldn't fret about introducing future friends to your kids, except to remember to discuss it with each other. I doubt there will ever be a situation that rivals the current one with Si. It's actually a good thing that kids have someone else they feel safe reaching out to, that is NOT mom or dad if they need to, especially with the amount of traveling you guys do. This is something that should be brought up in counceling.
Having once been in a similar position, I think the idea that Matt felt usurped is likely true. Once you've gotten to that point with an outsider, there's really no chance of being friends (beyond normal civility for a few moments) with them again. Regardless of what your spouse might think of them, no amount of effort will seal those wounds, except separation from them. And time. A lot of it.