Do i have reasons to be upset ?

I started dating this guy, let’s called him “A”, he works at the same place my sister works. I dating him for like 2-3 months. We had sex a couple of times. But he got a little intense, like talking about having another kid (he already have a daughter) getting married or move in together, stuff like that. so I decided to put a stop to it. I really liked him, but we wanted like different things i think, at the time. Two weeks later, my sister comes to me, she wanted to talk, she has a daughter (1 1/2 yr old) she starts : “i want to tell you I started a relationship and i am very happy”, i was like “ wow that is so nice”, then she continues, is “A”. I immediately started crying, and i ask her like “ did you know that is my ex,? That we were together for a few months? That we broke up two weeks ago, two weeks. She said “ I know he is the guy who can make me and my daughter happy.” I WAS devastated, is not because i was in love with him or anything, it was because she is my sister, and i think this is way out of line and plain disgusting. Two months later, she comes to me to talk again, saying she was pregnant, with his baby. I was out of words, am i overreacting? Our mother even accepts him, like he is her new man, new husband, future baby daddy of my future niece, yes, I remember telling her “is plain disgusting we had sex with the same guy”, and I told him “how could you do this? You know we are sisters” he said “things like this happen all the time.” Am i allow to be upset? It seems i am the only one who see all of this wrong.