I'm really hoping neither die though. Kenpachi is easily one of my favorite Bleach characters, and with this being about toughening him up, I doubt he'd die (it would make the fight pointless, but shocking). I always liked Unohana, and wanted to see her fight. So it would also suck if she died the first time she fights, while finally blossoming as a badass character. Instead of mother though, I'm guessing lover.
It's not like she looks older than Kenpachi. It could always be that they ran together before being shinigami (like Renji/Rukia). Or she discovered Kenpachi in the rough part of SS, in his usual manner he challenged her to a fight. He lost a battle for the first time, against her, and fell in love. Then she recruited him. It makes even more sense with her old name being Unohana Yachiru, and Kenpachi naming the pink haired kid Yachiru (unless I'm misunderstanding, and Yachiru is a title like Kenpachi or something).