I have given a bit of thought to Lucas's possible relationships with the others, very open to suggestions though.

[hider=RELATIONSHIPS]
[list][*]ROWAN: With four year age gap and Rowan thoroughly closed off by the time Lucas attends the Collegem, they didn't really have one. Lucas was far too caught up in his own life to give it much thought.
[*]SHERRY: Lucas definitely resented her, she's an example of someone with the natural talent he lacked, but worse, she's one of those spoiled half-siblings whose failure he blames for the harsher treatment he received. 
[*]ESTELLE: When he was young and life was still relatively pleasant, he would no doubt have been one of the younger children looking up to her. Between the new attitude his own mother took and age gap, I doubt they were close after she started at the Collegem. 
[*]ISANA: With the age difference and how withdrawn she became I doubt they had much of one.
[*]OPHELIA: One of the children young enough to have actually had childish fun with, but he never really shared her love of books. Do you think they'd have interacted much at the Collegem [@Rune_Alchemist] ?
[*]LUCILLE, ERYX: close enough in age to have seen each other as small children, but it sounds like their mother didn't take them to many family events. And if they wanted to fade into the background at the Collegem, then Lucas would have let them. Only one year between Lucas and Lucille though, and I think they might have got on if either of them had made the effort, but probably never did.
[*]RODIN: close age wise, and both used polearms, could have just been someone Lucas is compared to, but perhaps they could have practiced together and been friendly before Rodin left instead? [@Remram] 
[*]VINCENT As Zeroth said really. Fun times with Vincent would be one of the things Lucas really missed after turning 10. He wouldn't have done anything to correct Vincent's assumption, he blamed the stupid demanding family and mean tutors (as he saw them) rather than his half-brother.
Nothing else really came to mind yet[/list][/Hider]