I can't stop jerking off to my sister

My sister is 16 and I'm 20 we are from a poly family. Our daddy has 3 wives and I am the eldest son. I don't tell my daddy I have sexual feelings for my sister <br>I love her but I can't stop seeing her as a sex object. I am afraid if my daddy found out about this. I already asked my sister for a blow job but she said no, so how can I convince my sister to let me do sexual things with her?