How do i tell my mom about him?

ok so i have a bf but i am not supposed to and i know that if my mom finds out about him she would be ok with it but the only thing is is that she told me that if i ever had another boyfriend again she would send me to live with my dad but right now she has 3 daughters by birth and i am her baby and she doesn't have the other 2 they both hate her and so i want her to find out about my bf but i am afraid that if she finds out she will make me move so any help would be great ful and would it be better if i let her find out on her own or if i just come out and tell her? plz help