I left a really lovely guy because he was incapable of making us and our relationship a priority over his family and his exes (no shared children involved). Children and poor parental health often trumped our time together, but when it was a case of him spending hours on the phone, sorting out replacement washings machine etc, even paying for them for an ex, I about had my fill. When I raised the issue he wanted me to speak to her - err no! His family pulled a few stunts whereby arrangements made way in advance for a very close friend's special birthday, were completely ruined. No matter that I/we gave them advance notice that we would be busy on this particular date they then organised yet another "family outing". Due to a terminally ill family member we were doing things literally every weekend, which was fine, but it had been going on for some time, and this was one date that was about something else, albeit someone his family did not know. When he returned home after working away for the week they press-ganged him into agreeing to attend 'their' thing. I was fuming. Long story short, we both stayed home alone that night. Some people like to feel 'powerful', others like to be 'heroes'. These situations are hard when you refuse to be the one who shouts loudest, or steps in and plays the game as well.
My suggestion would be to not get hooked in. Make some plans of your own and make sure he knows it. It seems he has chosen her over you, maybe he needs to be needed, as my ex did, doesn't make him a bad person, just makes it very hard for you to remain in a relationship with him. Tell him what you believe is going on, and say if he chooses to prioritise someone else over you that tells you all you need to know about where you stand and it is not what you deserve. He may not intend to leave you for her right now, but it seems obvious that he would if it was an option. If you have to compete, it really isn't worth it. Dignity and self-respect have great value.
Leaving my guy was the hardest thing I've ever done, but it was the right thing, because there was always going to be someone more needy or shouty than me, and he chose a quiet life, rather than the one he actually wanted, because he knew no different. Sad but true.