God bless you for your efforts to help make your friend's birthday memorable!
You didn't say how long the plans have been made nor how soon the birthday is, but, personally, I think that the family should have planned their get-together at a different time/place since the 'birthday girl' had already made her own plans that involved 'the girls' (which INCLUDED the daughter/daughters-in-law who KNEW all HER plans)!
If I understand correctly, it sounds as if someone 'outside the loop' just got jealous of the 'girls' outing' and wanted to be included (badly enough to have EVERYONE go far away and pay hotel/motel rooms for themselves AND intrude on the already-laid plans!)
Does she love surprises? (I DON'T!)
So . . . what to do?
There's not really much you CAN do except pray that SHE has a wonderful time, and then make your own decision (about whether to go or not) by this criteria:
If I DO go and have a horrible time, would that be better than STAYING HOME and
a) 'deserting' your friend who may have a horrible time, also.
b) missing out on sharing a WONDERFUL time (if it turns out thus)
c) wishing you HAD gone (for any OTHER reason not mentioned).
Hope this helps (but, no, I DON'T think you're being selfish. They are).