Apologies but this is a long one...
Last year, just before my ds's first birthday, I accidentally discovered my partner had had another child (a girl) who was born 3 weeks before our ds. I only found out as he received a letter which hadn't been sealed properly and, being nosy, i opened it (I don't usually do this!). It turned out to be from the CSA chasing payments and the childs details were on the letter. When I confronted him, he admited that he'd sleep with this woman twice but, as it was during a time when we'd split up, it didn't count as cheating. We'd actually only been apart for 4 days and, in my opinion, we hadn't offically split up, just didn't see/speak to each other for a few days. I eventually forgave him and things had been okay until last week when his sister called. She has met this woman and her daughter a couple of times and they both (as well as other members of my partners family) think it's time my ds met his sister. After speaking to my partner, he has also said he'd like them to meet.
If i'm honest, I really don't want them to meet eachother but i'm not sure if it's just for selfish reasons. Do you think it's in his best interest to meet her?
Any advise would be really appreciated.