To cut a long story short: Wife's stepdad is arranging a surprise meal for MIL. He called at the weekend to extend invitation to wife/me/daughter but excluding foster-daughter who has been with us for two and a half years - 'you'll have to get a babysitter for her' because there's not enough room at the table. Now I can't go because how do I tell FD that she's not invited? AIBU to think (ILs and I don't get on) that this was a deliberate ploy to exclude me?