Have A Shower Thought

First of all, excuse my bad grammar because I'm not a native English speaker. Also don't take this too seriously this is just a weird random thought of mine!

What do you guys think? Is it still considered incest if not biologically related siblings develop romantic feelings for each other because they grew up as a family? If yes, should we consider a romantic relationship between childhood friends who grew up together and treat each other as siblings incest? Considering that the three of them (sibling a, sibling b, childhood friend) are friends since they were a child and treat each other like real siblings; In a situation where sibling b and the childhood friend developed a romantic feeling for sibling a, will it be more acceptable (morally right?) for the childhood friend to date sibling a compared to sibling b?

like vampire diaries for example, we have Elena being 17/18 year old at the start of VD while her love interest Stefan a 171 year old. That isn't right at all, yet because Stefan looks like a 17 year old people are fine with accepting it? No one questions it at all. Now imagine Stefan looking his age. Now think about this, some old ass guy wants to come and hang out with teenagers, this is what he chooses to do in his immortal long life. what?

Any Vampire Diary fans wanna explain this to me?