When my brother came out to my mom as bi, she just smiled, laughed and said "sounds good. Not really a big thing to me." And said she wouldn't care either way if it makes us happy.
I can't help but imagine their mom as an overly dramatic movie mom who spends 4/5 of the movie flailing and clutching her hair, running around screaming hysterically while everybody else ignores her completely until she passes out at the end from exhaustion.