"Then it's what we do." Because they must. Because Snow wanted it so badly she could almost feel the burn of it. She needed this chance and to give one as well. She needed to know the trust she gave at age ten wasn't entirely unfounded, that somewhere Regina still had a spark of goodness left in her soul.
She didn't want to see Henry or Emma hurt again. None of them deserved that much pain in their life. And in the end, everything for Snow came back to protecting her little family. Everything.
And for better or worse, Regina was, somehow, meant to be a part of it.