S.W.
I have been a stepmom for over 24 years to two wonderful young people. For the first 3 years of that time, I was not married to their father (then we were). My extended family - parents and siblings - included my stepkids as if they were my own from the very beginning. We attended family gatherings and visits whenever we could. I would have been disappointed with my parents/siblings if they had expected me to visit without my family along! My stepchildren's mother was unreasonable in many areas, but did not object to, and even commented on, how nice and accepting my family was to the kids.
Your ex and his "long-time girlfriend" and her son and your daughter are a FAMILY. Be glad that his girlfriend's family wants to be in your daughter's life. This does not take away from what you have with your daughter, it adds to her life. A child can not have too many involved, interested, adults in their life.
Please don't jeopardize your good relationship with your daughter's father by making this an issue. That you and he can relate well is the most important thing to your daughter.
BTW - I'm confused about how you state that you and your daughter's father were never married, and yet you seem to disapprove of his unmarried current relationship??