notlucreziaborgia
*You have no idea what this kid expects of her father, or what the dynamics are in any of her relationships. It may be that she quite simply doesn’t like OP and OP’s DD
Generally adults can indeed choose who they socialise with - be they family, friends or colleagues. She’s not merely been asked to socialise with her stepsister though, or even to be civil during an event they’re both required to be at (and this isn’t an event they’re both required to be at!). She’s being asked to have a sisterly relationship with OP’s daughter. She doesn’t want that and she doesn’t have to provide that either.
The DH doesn’t have a problem with his cousin doing this, so I doubt he’d have a problem with spending time with his cousin and family without OP and her daughter*
But we do know what this child expects. She expects 100% attention from her father and mother and for her step sister and her fathers wife to not exist.
No one is expecting this girl to be sisterly to her step sister. It is a wedding. She can ignore her step sister if she wants to.
Whilst no one but the bride and groom need be at this wedding but if they are going to invite family and friends then if you invite a cousin and his wife and cousins dd then you also invite cousins step daughter that he lives with.
If the step father doesn’t have a problem with what his cousin has done and continues to go to these type of events alone without his wife and step child then eventually he will either stop going or stop being married.
I suppose he has to come to a decision which he values more. Family birthdays and weddings or Family birthdays and weddings.