If you do commit to a ride once a week...can you keep the bike at your place so it is not a constant reminder to your dad that she has it?
Put your little tyke in a trailer behind you and then it truely will be a "Girls Family Outing!" AND...it will give your mom something to report back to your dad about (what the little one does, says....) go to a park so her stories will be about slides and swings...not bikes!
I hope this all works out for you and your mom...I think you need it as much as she does!
Hope you are happily pedaling with your mom soon!!!
I am a nobody; nobody is perfect, and therefore I am perfect.