I don't think "willpower" is all that well-defined a concept, actually. It can mean a passive ability to endure difficulties; and it can also mean an active boldness to shape one's situation.
I see the former as included in D&D Wisdom, and the latter as included in D&D Charisma. And the latter is surely more appropriate to the 5e sorcerer.
As for how the mental stats fit together, I've always mapped the mental stats onto the physical ones like so: Cha is mental Str, Int is mental Dex, and Wis is mental Con.
EDIT:
A further thought. Sorcerers could use Cha to shape spells, and if there's ever a check to prevent themselves from losing control, they could use Wis for that. I like the idea of sorcerers having to hold a spell together and keep it from going wild - wizards use elaborate symbolic structures to do that, but sorcerers have to do it all in their head.