"I..." she flustered, young and unsure, her shining blond hair adding to the illusion of innocence. "I wanted to offer my help. My father was never able to finish his work, and I can't do it alone. I thought that maybe, if you had a spot on the Brotherhood of Mutants, you'd let me join up. I want to take down the X-Men, and I thought maybe you could use my skills."
There. That was all true, mostly. Maybe a few important omissions, but the important points were true: she wanted to sign up for the Brotherhood and infiltrate the X-Men, in order to destroy them. Simple enough.