I'm sorry your vacation has gotten off to a stressful start. I can definitely relate to that alien-like feeling around people who I don't relate to very easily. Sometimes I assume that it is my fault or my own fundamental weirdness that is causing the disconnect, as opposed to it just being the reality of the situation that a particular person and I don't relate well to each other... or that it could even partly be the other person's issue. I think that depression gets us to believe that we are alone, alien, other. But, your partner's sister's issues are not your fault.

You mentioned feeling more "free and comfortable" when you are alone. I think it is a gift in some ways to be comfortable in your own skin and be able to be alone. Lots of people aren't. But, I'm also wondering if there are people (like maybe your partner?) that you do feel free and comfortable around? I'm definitely an introvert myself, and people can wear me out too... but those few close friends make all the difference. Just a thought.

I wish you well surviving the rest of your vacation. I hope you post again and let us know how it is going.