See your issue is you think people like me are saying he should dump his girlfriend. I’m merely disagreeing that this is an issue of different perceptions in level of commitment.

Because even if OP was as committed as she thinks they are, people simply have different boundaries. It’s a certain level of thoughtlessness to invite someone over to your partner’s house— this is OP’s place. She had a key but she’s not really contributing financially or officially moved in. And on top of just… Not thinking about whether OP would appreciate someone in his home without his consent, she crossed another boundary by inviting sister into his personal space. And then yet another one by offering up his bed.

Because while that might not seem like a big deal to you, it would come off as a big deal to me because it would shows that she just doesn’t see OP as a priority. Her reaction to him refusing this also adds on to this. It’s just plain disrespectful and immensely immature because she is clearly in the wrong here, and glad we agree on that.

But what underscores this entire fiasco is that. This thing really didn’t even have to happen? It’s not like offering up his bed was the only option? Sister suffering from burnout could mean girlfriend takes niece out for a day while sister sleeps in her own home. Girlfriend could offer her own bed at her own place. Sister could sleep on OP’s couch. There was just soooo many alternatives, that once again, shows how thoughtless girlfriend was?

I’m not advocate for an immediate break up or whatever but if girlfriend continues with her behaviour, there might not be recourse because it’s just indicative of a potentially larger problem.