I hope you do get to take your dad out alone. Maybe you can send your step mom to the salon for some girl time only activity lol.
Ok... this is partly lame dysfunctional advice... but this is what I've come to in a smiliar situation...
1) have no expectations or "hope" that things will be good... thus when you are already flat on the floor, you have no further to fall with disappointment, somewhat less painful
2) just say "no" and don't go / or do it, activity xyz
3) if present, close my eyes and tell myself that this must be some life character test of strength, and what can I get out of it to better myself in an awful situation
Good Luck!

