Trying not to be petty but.......

I know we are all protective of our new babies and I am trying to forgive my stepson but part of me can't let go. I had my little one this month and a couple months before she was born I found out my 17 year old stepson (who lives with us) and his mother (who only sees him every couple months) were saying things about how his dad better get a DNA test for the baby and he was saying she's not his sister, etc. I was so hurt that he would say something like that, not surprised by her though. We were four days away from seeing a doctor to get our fertility tested when we finally got our positive! We tried so long for her. Well when his dad confronted him about it, supposedly it was discussed that he owed me an apology, I never got one. Now that the baby is here, he's all over her, and always talks about his baby sister, etc... Now I'm trying not to be petty but I have some sort of resentment toward him for the things he said. It irritates me beyond belief because all I can think of when I hear him is how he was saying she's not his sister. I know I need to just forgive and move on but I haven't figured out how. Do you think you could easily just let it go?