What must i do?My step brother wants us to be lovers…help

step brother wants me

My mother re-married over six years ago and we moved into my step-father’s house. He has three children and I’ve always found his eldest child good-looking.
He is in the university and I’m at the nursing school. I’m 23 and he’s two years older.
About five months ago, we finally agreed to having mutual feelings of love and we kissed a lot.
We recently took things a step further by making love. I love him so much and he feels the same.
Our parents don’t know what is going on and I’m kind of scared already.
He says we have nothing to fear as we are not related by blood, that legally, there’s nothing stopping us from getting married.Please, is he right? Can we go on doing this?