Where to start....
The song is clearly about someone he cares very much about. It is about someone he is no longer in a relationship with but someone he was once very close to. I’ve always felt like he is establishing that this song is about someone he cares about deeply, his sweet creature, and yet someone he was no longer in a romantic relationship with at the time the song was written. He had been talking to either this person or someone else close in his life about where this relationship failed, and went wrong. That being said, there is hope. Stating that they are still young gives the impression that there is still time for them to figure things out between them, and while they don’t know where they are going in their individual lives, there is a sense of belonging to each other.
"And oh we started, two hearts in one home" - Harry has only ever lived with his Mum (and Dad for a while), his sister and Louis.
The song does not feel platonic or about a sibling relationship.
"Had another talk about where its going wrong" - that references a romantic relationship. It should also be noted that Harry did make a sort of denial about it being about Louis (no surprise there) but then I heard an interview where he said the song was about one person, but he was not sure if the person knew it was about them, which to me makes no sense if it were in fact about Gemma. He would simply tell her it was about her if it was indeed about her.
In the early days, Harry used to refer to Louis as 'sweetums' and 'sweetcheeks' - plenty of tweets still in existence to attest to that. He is his "Sweet" creature
As the song seems to be about a broken relationship and also talks about how they started under the same roof, it makes no sense that it would be about a sibling. There is no broken relationship with Gemma and the other person he lived under the same roof with from the X Factor days was Louis Tomlinson.
He acknowledges that he and this person argue, largely because they are both stubborn, but…
"When I run out of road, you bring me home"
…despite the arguing and stubbornness, this person represents home to him. They not only lived in a shared home, but this person somewhat embodies what “home” means to him - safety, comfort, where he can be himself (”for your eyes only” comes to mind). This person holds a place in his heart and will always be his safe space.
Thank you for coming to my TED talk. I could write more but I am tired and it is all pretty much there. This song is romantic and not familial.