Since Kevin might have lost his mind but still loved Gwen (although he almost murdered her, more than) after becoming Ultimate Kevin, I'm surprised he married someone else.
And then I remembered Sunny was (a little) interested in him while she was still dating Antonio. So maybe, in his power-hungry state, Kevin got engaged to the closest person that resembled Gwen: her carefree, powerful and full of mana cousin.
Then they got Devlin, and because she was still a bit immature, she didn't make sure he wouldn't end up as much of a "bad boy" as Kevin but was considerate enough not to tell him why he was in the Null Void.
And when Kevin came back, she probably didn't think he would make so much of a trouble, so she let Devlin go to him.
Anyway, that's my idea of how it could possibly be Sunny, what do you think?