Everything I can think of that may be relevant:
-Setting is modern day US/Canada.
-Birth mom's partner was raised Modern Orthodox, but isn't very observant now. The sisters were raised Reform but went to Orthodox schools. Birth mom, like her partner, isn't particularly observant, but adoptive mom is very traditionally-observant for someone who's Reform (though she's also very liberal in many ways).
-All three moms are from a middle class background, though birth mom and her partner are now quite well-off.
-Birth mom's partner is an officer in the military.
-Both sisters are huge sci-fi fans. Doctor Who and Ursula K Leguin are a particular favorites. I know Leguin has some families with four parents, but I couldn't remember if she ever said what the kids call their parents.