Op again. I am close with both Ann and Belle and our other siblings though they live further away yet.
I think the issue I have had is that Belle will sacrifice things she wants to do to be there for Ann. She has stated several times She feels like she needs to since those tulip close by each other. She complains to me quite frequently about Ann's parenting choices but feels the need to constantly help out. I'm older than Belle so perhaps I feel a bit more protective of her. But you're right, she is an adult who can make her own choices.
I am not jealous of their relationship and to be honest I would find it quite suffocating. I guess I'm happy. I love a couple of hours away but have grown frustrated over What I have perceived to be and taking advantage of Belle and Belle enabling ann. But you're right, it doesn't involve me so I will keep that in mind.

O.K. a couple of things. You describe the two sisters as thoroughly enmeshed, perhaps to an unhealthy extent. However, rather than being thankful that you're not equally enmeshed, you seem almost jealous of the closeness of their relationship with each other.

Feeling triggered by what they are doing is externalizing your locus of control. You should try to be aware that these emotions, when they arise, are internally generated, and not to outsource them to your sisters. This will increase your feeling of emotional stability and control.

As for the practical issue of scheduling events, I think the best you can try to do is just try to schedule family gatherings at the house of one or the other of the enmeshed sisters. Bring all the sisters and all the kids. More complex arrangements seem to yield repeated disappointments, so lower your expectations about those kind of events, and get more realistic. Be satisfied that they are what they are and accept that as "good enough."

Dad gets to play video games on Saturdays while Mom and Auntie do the laundry and child care? And then everyone still calls him an involved father? I understand taking some downtime, but spending Saturdays on video games and gym time while other people take of your house and home is quite a privilege.

Agree that OP needs to take a step back (which she clearly already knows), but I would be annoyed by this dynamic too.

I think their relationship is weird as heck and I wonder what their husbands think. I would be pissed I my husband was basically raising our kids with his brother.

I am the childless sibling and I don't see anything wrong with their dynamic....?

I think you feel left out. I am one of three sisters. My other two sisters are closer. It kind of sucks and often hurts my feelings. But that's also because they say mean things about me behind my back while being all buddy buddy with each other.

They are not codependent they are interdependent.

You are independent and have your pride tied up in that and you are mad about their interdependence because you don’t understand it.

You’ve learned that being independent is the best way to be and anything else is codependent but you are wrong.

Their relationship is healthy. They have a bond that you don’t have. Ann is bonded with belle’s kids in a way your kids will never experience , that hurts you.

You are on the outside looking in.