I went with someone for 7 years who had a daughter and their relationship ruined ours. I thought things would get better but they got worse as the child got older. I was also pushed aside and treated so badly by this kid. One time I cooked dinner at my place and just had new rugs installed. The darling deliberately "spilled" her milk on the rugs and "dropped" her plate of spaghetti - all the while smirking. Her father did not say one thing except that it was an accident. Believe me, it was no accident.
The 3 of us would go on vacation for a week at the shore each year and they would go bike riding early, never letting me know ahead of time so I just sat and waited for them to return. I was basically there to be their maid.
I sat down with him and voiced my concerns and I was told that no one would come between the two of them. He gave me his answer that I would never be involved in their lives. I left that relationship after that.
I could go on and on. It is tough, it wears you down, and strips you of everything.
I wish you much luck but in these situations I think it is best to get out of the relaionship as there is nothing but heartbreak and you will always be an outsider.