I personally would not engage in it, regardless if its a step-bro/sis or biological one it just....does not sit well with me.
Doesn't mean that I don't think others have the right to engage in it, as long as both parties consent...I think it is unfair that is illegal in most countries for incest to practice sex even if they are both old enough to make their own decisions, the only valid argument there is for it is the babies might be deformed which is a good argument but still, I feel they're not hurting anybody therefore the law doesn't have the right to tell incest couples they can't be together or do stuff together...