It's also probably relatable to a significant portion of the audience. I wager a fair number of people have experienced a relationship or wanted a relationship stemming from that situation.

Through no choice of yours or theirs, suddenly you are spending a lot of time with someone. From some perspective 'relatives', but pratically speaking you have a stranger who is your age move in and you are both 15 to 17 and things happen.

I had a not-blood relation suddenly happen when we were both sixteen, and we ended up dating for a bit. A bit more removed than 'siblings', but close enough to spend a lot of time together.