Honestly, I don't think Sunny is a villain, I just think she's misguided when it comes to her family and her life.
I have a friend who RPs Sunny as a main for my Gwen, and she portrays Sunny so beautifully and shows deeper, hidden sides of her character that the franchise did not show.
Therefore, it is my personal belief that had Sunny joined the team and had gotten to know Gwen better (heck, maybe even teach Gwen some new Anodite tricks/abilities along the way), then they could've grown to be like sisters/best friends to one another.
I feel like Sunny is how Ben could've been without Gwen around and if he was the one with mana powers. She surely has more depth, but she can't tap into that the way she raised and grew up. If she spent more time with the team, she'd learn better, but she's gone a bit too far for Ben and his friends to have patience with her (even though they weren't much better in the past)