My Sister Is 11, Her Best Friend Is 11, AND PREGNANT?

Omg idk how to approach this. I take care of my sister and her best friend is 11 yearss old and pregnant by a 15 year old. This is gross and ridiculous. I don't wanna separate them cuz my sister doesn't really have any friends and she's the only friend she has. But I just think she's kinda a bad influence on her. Like, that baby will be 3 when my sister is 15. I don't want her to get pregnant at such a young age. Help? Advice?