I agree with Craig.
I think Peter and Tracy have one of the most believable sibling relationships in Soap re how they feel and act towards each other as I alwYs yhink it is clear that they have great affection for each other and usually het on bit have no illusions either. Basically they don't always like each ither but yhey love each other, know each other better than anybody else and will always support each other. Them not being blood relations makes that close relationship all the nicer.
It's also worth remembering that although they might not be blood, Tracy was only 4 and Peter 16 when Ken and Deardre married so they've been in each other's lives for a very long time for that sibling bond to form. Its hard to belie e they'd ever see each other in a sexual way , especially Peter who will remember Tracy as a little girl, barely more than a toddler.
Not to mention the fact that as Ken adopted Tracy, they aren't just step siblings but full siblings legally. Which means it would be incest and therefore s criminal offence, evebn though there is no biological link.