What shud i tell her?, 15 & pregnant?

me and my bf have been trying for a long time to concieve a baby, and my younger sister lives with me and him, bcoz our parents died a few years ago, im currently 6 weeks pregnant :) and my little sister has turned around and told me the only reason she tried getting pregnant, was to give her baby 2 me and my bf :O, i am rather confused on what to do, as she is 8 weeks, and is saying she doesnt want 2 keep it if i dont take the baby...... any help would be nice, plz nothing harsh :)