Get really excited and tell her you want seven more kids
She's expecting you to freak out/get angry. The best thing you could possibly do is be totally down with it and act like you are ready to take that life on with her. I feel like that will catch her off guard more than any "I got another girl pregnant" could ever do.
"Goddamnit! First my sister, and now you?! Time for suicide."
OP is a fucking boss