The concepts of Whitney having an older or younger brother are both good and can add to his gap moe appeal. Sorry for my own rambling from this point on! An older brother could be an abusive figure who Whitney both idolises and fears/secretly detests, currently in a prison or YOI. Whitney would probably downplay the bullying and abuse and treat is as a joke and just roughhousing until later in life. I've always hc'd him to be related to either scarred or tattooed inmate from the prison. Alternatively, with an younger brother, Whitney could bully him and even lash out at him after nasty arguments with their father sometimes, but he would also get violent if anyone else dares to bully his little brother, as it's his own blood after all. For younger brother's personality, a mini Whitney is cute, but a quiet kid who's avoided by the bullies only thanks to his older brother's reputation is even cuter, he would also be one of Whitney's biggest vulnerabilities.
I like the way you think! I always thought if Whitney had siblings they'd bicker alot but would also be very closely bonded because of the abuse they have to endure. I believe Whitney as an older brother is a dipshit but he would subconsciously take on majority of their dad's shit to spare his younger sibling(s). I also like the idea of little bro Whit with an older brother who removed himself from the family dynamic (thru prison or moving out) and I like thinking about all the emotional turmoil that would leave Whitney with (something he wouldn't reflect on normally or refuses to)
Revospring uses Markdown for formatting
*italic text* for italic text
**bold text** for bold text
[link](https://example.com) for link