Im pregnant with my sisters husbands baby?

they arent married yet. but there getting married in 2 weeks. and 2 months ago i had sex with him and got pregnant. and him and my sister have 3 kids together. and its gonna be weird. i think.....do i tell him or her its jakes baby or say its somebody else's baby.
Update: im only 15. Jakes 24 he'll get in trouble
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