Oh OP I went through similar - my older brother physically and emotionally abused me well into our early twenties until I finally cut contact. In our case it was undiagnosed MH issues with him that was driving a lot of it, though by the time I was an adult that was of little comfort.
I had a two year old DS when I became pregnant with DC2, a girl. I cried and cried when I found out she was a girl. I was absolutely terrified of it happening all over again. I was so conflicted over my feelings about my beautiful, lovely little boy and terrified for my unborn daughter.
My husband spent a lot of time talking it all through with me, telling me I am not my parents, he is his own parent who will not let that happen, that my son is not my brother, that we are a new family.
We are now a family of a four year old boy, a two year old girl and another girl on the way. My children adore each other, though that's not too different to how my brother and I were at this age. However we talk a lot as a family about how we look after each other, care for each other, that in this family certain things are not acceptable - we have a family statement up on the wall about what we can expect and not expect from each other. It's positively driven but still, we hope to reinforce that we take care of one another and help each other, that to be part of this family is a special thing.
But I don't think the residual fears have truly gone away and in my heart of hearts, I do think part of the driver in having another DC, whatever their sex, was to create a different dynamic to the one I had. I wish you well for the future OP. What we both experienced was not a normal sibling relationship, I do know that much 