It's been said already, but I'll throw my two cents in anyway.
Go because you need the closure (whatever that is....
) or go to help out the part of the family that isn't worthless, but don't go because you feel it's expected of you. You're under no obligation to this woman (or the rest of the family for that matter) just because you share some relatives..somewhere.
I fear that anger and umbrage are getting kind of a bum rap here. I really don't think that there are any 'negative' emotions; just emotions. Anger, just like everything else in life, is fine in moderation and when it is directed by real purpose.