Yes, I would say that would be ok.
Distasteful perhaps, but ok because they are not from the same biological family.
So let us take the following two scenarios:
S1:
A man and woman get married, the man already has a one year old son from a previous marriage; the man dies soon after his remarriage leaving the wife to raise her step-son as a single mother; when the boy reaches the age of consent and both he and his step mother are willing.
S2:
A male and female sibling are separated soon after birth, upon reaching the age of consent they meet each other unaware of their blood relationship they become intimate, later finding out that they are related - they do not stop their relationship, but take steps to prevent pregnancy.
Is S1 acceptable (if in your words distasteful)? What about S2? Is S1 more acceptable than S2? And most importantly, WHY?