He doesn't really want to tell this. There are reasons he's held it in so long.

And it's worse to tell it to her, now, in this context. With what she's just said about phasing. But it will never be any easier, and she won't thank him for waiting; Kate has always preferred to get things over with.

"He was a mutant, and a magician. Rasputin. He learned from Sinister. When he died finally, at the end, he put his essence into all his descendants. Babies."

"He meant to be reborn. But he needs his power to not be split among the bloodline for that to happen. To only have one left alive."

He wants something to do with his hands.