You’re triggered because it’s affecting you, and that makes you think about other ways it could be handled and delve deeper into their dynamics than if it was some distant happening that didn’t affect your life. A & B have their own thing going on, and their husbands can live with things as they are or learn to manage their relationships. It sucks to watch people you care about do dumb stuff but there’s absolutely nothing you can say that wouldn’t come off as manipulative that could even have a chance of changing their dynamics. If B can’t handle simple choices like prioritizing her marriage over babysitting her nieces and nephews, maybe she’s not really ready for parenthood. Her husband presumably sees what’s happening and can choose to wait for her to quit delaying starting their own family because she’s too enmeshed with her sister’s family, or he can choose another path. If he’s on that particular fence, I doubt another sister inserting herself into their relationship will be helpful.