What is the etiquette when you don't like your long-term partner's new partner? My girlfriend just got a new girlfriend and she's not a bad person or anything, just not the kind of person I get on with. Should I talk to my girlfriend about it and potentially stress her out a lot, or just leave it?
It’s totally fine to just not click, personally, with a metamour. The etiquette is about the same as when you dislike someone you work with, or someone a family member is dating - be as civil and polite as possible, don’t create unnecessary gossip or drama, and don’t overextend your capabilities.
Politely decline invitations to hang out solo with her, find ways to occupy yourself when she’s around, and don’t try to force yourself to like her. Be careful about ‘stewing’ on things about her you don’t like, and try to remain as neutral as possible. Not everyone in the world is going to be someone you get along with well, and that’s totally fine!
I don’t think you ought to tell your girlfriend you flat out don’t like her new partner, but be gently honest about it when it comes up. If she invites you on a three day camping trip with her and Baldurina, you might say “I think that would be a lot of time for me to spend with her - but you two should go and have a good time!”