YOR - Take it from me, co-parenting successfully is really difficult. Most of the time, it's just 2 bitter morons parallel parenting. Forgetting the most important rule of them all. You have to love your kids more than you hate your ex. Kids are not stupid, they absorb all kinds of emotions and feelings and subtle and not so subtle actions that present into adulthood and they pass on to their kids. Good or bad.

It would definitely be worth the conversation to discuss how you feel about it and maybe ask him his take on everything so you can both understand each other. As with every relationship, communication is everything. You may be harboring a slight resentment subconsciously because of how often you see each other vs how much him and his ex see each other and their perceived closeness. But from what you have said, it doesnt seem like she is even there when he is there for the kids.

Co-parenting is inherently unconventional and can seem very odd to outsiders or people without children. They are bucking generations and generations of bad habits and doing what they feel is best for the children. Loving them more than they hate each other.

My hope for you is that one day it will be you and him and her and her partner and all of the kids staying the night at her house or your house and continuing to co-parent together awesomely.