im going to throw in a different perspective.
My mother died and my father remarried very quickly. SM had a daughter my age and GD of 10.
I adore my SM ....bit of a thoroughly spoiled GD but thats by-the-by.
My SM made my father very happy for 7 years. Cant say my SS is a sister, but she is a very good friend.
Parents arent 'children' we can control.
I also have issues that my Dad & I used to be very close & he has lost interest in me & many of his old friends since he started this new relationship/got married
I can see where you are coming from - once you were the Princess in his life, now you have grown up a nd got your own relationship - maybe he felt the same way once - you moved on with a new man in your life and didnt give him (dad) as much time as you once did?
You need to make a friend of your new SM. Women always control relationships. In a way, its lovely you have a step sibling the same age as your own child - they can grow up together and you can merge families because you have common interests.
I suppose I've been in your situation - but I had a very good older female friend who showed me i was just being jealous of my fathers new relationship and the time I no longer had with him.
As I say, I love my SM to bits, she made my father happy and that in turn made me happy.