Have NC. I have written this post and deleted lots of times- finding it v hard to post and trying to keep it short and not put in too much stuff to justify myself.
When I was at sixth form college I had a horrible boyfriend and low self esteem. I also had a male friend (MF), who confided in me his girlfriend (I'll call her Sarah) was pregnant- he was panicking. We got drunk and slept together. It was a horrible thing to do and I wish I could give my 17 yr old self a shake. Everyone at college and in my town found out, including Sarah- I become a social pariah and was v happy to leave town and go to uni miles away in the north.
A year later, MF came to the town where I was studying, for work. His DS had been born, MF was still with Sarah, and he would go home to visit them on weekends. However, they found long distance too hard and split up after a year. Sarah lived v near MF's mum, and saw her daily, and was v close to other family who were of course v involved with DS, their grandchild, and MF kept returning to hometown to see DS regularly.
When MF had been single for about 6 months we got back in touch and began dating. After a few months MF told his parents and sister we were together. Sarah found out, and understandably was v angry. She told MF's sister what I had done years before. Things were very difficult, but after a couple of years of me dating MF, Sarah agreed I could meet her DS (then 4).
Years later, MF and I are now married (I'll keep using MF though for continuity). We have DS EOW and are happy- DS and I get on well. However, Sarah is of course still a big part of the family and very close in particular to SIL.
My worry is that SIL doesn't like me, because of what I did to Sarah all those years ago. She is pleasant but there are lots of small cutting remarks (inc Facebook, sorry), or things we are left out of. ILs are generally a very close family, who holiday together etc, and I/we are treated noticeably differently.
My fear is that my relationship with SIL is ruined forever, and that MF is left out of family things because of me. I also worry that when we have children they won't have the same relationship with their cousin that my DSS does.
Can I do anything to try to repair things with SIL? I am so sad and angry with myself that maybe I ruined my relationship with my in laws by one stupid night in sixth form