step mom advice

J
I am 8 week pregnant and I have been with my bf for almost 5 years. My bf had a son with a girl he used to hook up with when he was young. Tried to make it work but couldn't. After 5 years she still does not look at me and fills her sons head with lies about me and his father. 
I met my bf 2 year after they split and we have his sons 1/2 the time. His son and I are best buddies and I absolutely adore him. He is now 7. He has been begging for a little brother for the last year and I know he will be so excited to find out. 
I am not telling him I'm pregnant until 12 weeks to be safe but I am terrified of what his mother is going to put in his head when she finds out. Or how to deal with the her blowing up my bfs phone when it is really not her business. She already tells us to have a kid so we forget about hers and says I should disappear because I will never be his step mom.
We talk about it and he asks why his mom hates me. I say I don't know and that I have never talked to her and that maybe one day we will be friends. I also say as long as you like me that's all that matters.
I really need some advice from any step moms who have been there. I don't think it's okay that she will be taking away from what should be an exciting time for me and would really love some advice on how to handle it all. 

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