Well if it wouldnt hurt you emotionally to contact him and not get a good reaction then i would. It may be just what his sister said and maybe he is worried that you dont want him to. It wouldnt hurt anything just to maybe send him an update and tell him that anytime he wants updates then he can contact you. That just bc it didnt work with you two doesnt mean he is cut off from his child. That way you did your part and if he doesnt do anything from there then it is on him
I was in that boat, yes as much as it kills you don't message him. If he cares&wants a part in your child's life he will message you. As for him having another child, I have seen that before, he has one he has no contact with, moat chances are&very sadly to say he probably won't change. As for the sister she should he messaging you to talk to you. My best advice is to hold your head high, you will get through it, it is tough I know that. But you have friends&family who love you&support you&will help you every step of the way.