Woman has known man since he was born. Their parents are best friends and so for years we're practically brought up together.
Then aged 9, they went to different schools and had ceased to see each other at all.
Er... well to the age of 9. People change massively between the ages of 9 and 27. These two people (who are not the potential couple in the story) are NOT like brother and sister. They are two kids who knew each other years ago and have had no significant relationship. A nine year old will barely remember anything really. It's no big deal.
This background is irrelevant
Aged 27, woman meets the mans sister and they become close. She introduces woman to her brother and after several months there seems to be an attraction from both.
How on earth if they were "practically brought up together" has this women never met the man's sister before? Either because (a) this is a crock of shit or (b) the man's sister was born afterwards ergo making the background above EVEN more irrelevant
Woman is in a similar profession to the university course man has just started.
So what? Again seems irrelevant unless she is teaching him.
Only problem is, while man is very mature, he's only 18. She is 27 and 28 in a few weeks.
Wrong on many different levels and an absolute no go?
So user1488318718 what are the many different levels you are thinking of?
Seems to me there is only one possible one which is the age gap. Whether it is "wrong" depends on whether it is wrong between these two people.
If he wants to sew his wild oats, and she want a husband and children = wrong.
If they are both just having fun and attracted no strings attached = not wrong.
If he is seriously mature for his age and they are compatible and both want a serious relationship = not wrong.
Just depends on who they are and what they each want and how compatible they are.
I don't get how the backstory waffle is relevant here at all or what the "many levels" are.