Ok so ive know this girl since i was like 6 and ive constantly had a thing for her but the thing is she my step dads little sister shes a year younger than me and in our teen years i decided to "man up" and tell her how i feel she said she felt the same way so i kissed her and eventually we kept doing this whenever we got the chance and eventually it lead to sex .... but during that time she had a bf but she decided to stay with him and even now we will hang out and sorta act like a couple and ive fallen in love with her but shes dating another guy again and it bugs me but my wuestion is should i still do this with her or just leave

They're not blood-related.
It's not just that either; because she doesn't break up with her own boyfriend yet still is sexually active with him, she is probably just using him anyways.
And yes, she's probably using him.
And OP, if you're OK with being used for sex, keep it up.
If not, break it off.