Last year my sister asked us to go with her to a concert. We have no interest whatsoever in the band or in going to this scale of concert (it's in a stadium so huge) and would have to travel an hour each way to get to it, plus we'd have to leave the kids in a relative's house overnight, which they wouldn't like at all (they're very little and would still be very nervous of staying somewhere without us). We agreed to go because she has no-one else to go with and wouldn't have gone alone. My father paid for the tickets for us as our christmas present.
Since we agreed to go, we invited her on holidays with us. We changed the dates from nice quiet dates to busy ones so that she'd be able to come with us, and invited her over every week to plan it with us and watch related movies with the kids. We made her dinner every week, gave her a key in case she got there before we got home from work, and bought her slippers so she'd feel comfy and at home (and explained that it wasn't that we didn't want her wearing shoes, that she could wear whatever she felt comfiest in, in case anyone's thinking we were being the footwear police!). She came for a few months but then stopped and we didn't know why.
We have since found out that she's been complaining to my parents that we made her feel terribly unwelcome, and that was why she stopped coming. She didn't want to see DH and I, but made arrangements to go to my parents' house when they had the kids so that she could see them without us. My parents have told me I'm a horrible person over it all, she thinks we're horrible and the atmosphere in the family is generally excruciating.
She still wants us to go to the concert. WWBU to not do her this favour? I know we said we would, but it's become obvious that we really don't have the relationship I thought we had and I can't face the awkwardness of it all, or the upset to the kids for the sake of someone who'll just end up bitching about us behind our backs.