My sister has been sleeping with my baby daddy?

Me and the father of my son have never been in a relationship we were just friends but we recently began dating I didn't tell my family because I wanted to wait and see how things went. Well my older sister found out and she went nuts because apparently she and my sons father had been sleeping together. And to make matters worse she was mad at me. I feel like I have done nothing wrong she crossed the line. I am so upset I just don't know how to handle this I not spoke to her since this happened it has only been a couple days since this happened. I thought we had a good relationship but I guess I was wrong. So I guess my main question is why would she do this and why is she mad at me because I was seeing my sons father? I also need advice on getting over the hurt and anger I feel I don't want to lose my sister but I just don't see our relationship ever being the same.
My sister has been sleeping with my baby daddy?
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