I'm going to a non-refundable couples massage with my gf instead of helping my sister move.
Why are you doing that to your sister?
I think it's way better to go to the massage with my girlfriend than moving my stuff with my sister, who's demanding and annoying.
I see your point, but it's wrong to dishonor your commitments you made with a sibling. It's always good to help your siblings when they are in need.
Hmm.. maybe.
The thing is you made a promise to your sister. Also your gf might judge you partly by how you treat your sister.