I offered to mind db and sil's children recently when their child minder was sick. I am a sahm so took my baby with me and left older children in the care of dh and neighbors and went to London (I'm in Bristol) to mind dn's. They are easy children and so it was not an onerous task. I was there for 3 nights. On my last evening sil came home and immediately popped out ("for 2 minutes") to collect something from a neighbors house before dinner. db and I waited and waited (for her to come back so we could have dinner together as I had already fed the children) but after two hours we went ahead and ate. She finally came home 3 hours later (10pm) having gone to a bar for drinks with the neighbor. I am really insulted or hurt (not quite sure which) as I thought we had a really good relationship. We have always gotten on very well but now I realize (maybe over-analyzing it) that she has made points in the past about how I am not her sister. Well of course I'm not but I thought it would be respectful to have said to the neighbor something like: I'm sorry I can't come out, sil and dh are waiting for me....
I really want to get over this as we do have to be in each others company at family events and I don't want to harbor resentment so tell me IABU!! (or does it sound like she probably doesn't even like me and was desperate for me to leave)