Should I try and be friends with my boyfriend's ex if...?

[neko]

Well this ex of his and me used to be friends. Not close friends, but we'd talk on facebook and in person when she was at the mall where I used to work. I just found out this week I am 3 weeks pregnant. My boyfriend's ex had a daughter with my boyfriend who is now two and will be three by the time his and my child is born. I'd really like it if my child could have a good relationship with his/her big half sister. Reason hers and my friendship ended was because she flipped out when she found out him and I were together, even though she and him broke up nearly two years ago and she left him for his now ex best friend. She also said I had to break up with him because she had him first (which in a sense isn't true because him and I dated back in 9th grade but broke up because he moved) and he didn't even know her until a few months after he moved to his new school.
So..what do you think?