I so agree with you on the idea that Katana_Gelder should just do what she wants.
But I think that expanding into other topics ("do you mind if I don't come?") that haven't actually formally been raised is a form of anticipating other people's conversation, and I don't think it's a smart tactic in a situation like this.
And in fact, in -this- conversation, there hasn't been any drama, so anticipating it and trying to send a message that you're not going to participate in it, is borderline rude--don't assume drama or rudeness on [my] part until [ I ]'ve actually done it, thank you [brackets to indicate, I mean a generic person]. I personally, as me, have had that done to me, and it takes me from non-drama to the edge of drama--instead of focusing on the pure facts ("are you going to the family lunch?") suddenly we're talking about whether I'm going to be upset--and I wasn't even upset in the first place. So, well, I'll -become- upset if you want me to!
The OP doesn't know why her sister is asking. Maybe once she hears "no, unfort." she'll think, 'Hmm, well, I guess I'd better go to Dad's lunch after all.' Why derail that? Sure, it might not happen, but give her the room to either be rude or not be rude.
So if a person wants to soften, which you and I both think is more comfortable, I think it's best to soften with blather ("unfortunately") instead of by bringing in some other actual issue ("do you mind" or something else).