I probably knew this once, but while googling for "Valentine" references in Shakespeare's work I re-discovered that Mercutio has a brother named Valentine:
Stay, fellow; I can read.
'Signior Martino and his wife and daughters;
County Anselme and his beauteous sisters; the lady
widow of Vitravio; Signior Placentio and his lovely
nieces; Mercutio and his brother Valentine; mine
uncle Capulet, his wife and daughters; my fair niece
Rosaline; Livia; Signior Valentio and his cousin
Tybalt, Lucio and the lively Helena.' A fair
assembly: whither should they come?
Romeo and Juliet – Act 1, Scene 2. Lines: 65-73.
shakespeare.clusty.com; February 13, 2014
Now, clearly, I'm a bit stuck because I've been slamming the new Rosaline movie as having nothing to do with Shakespeare. If anything, we're at least given a bit of character for Rosaline by Shakespeare - Valentine gets nothing but a name on an invitation! So anything you say about Mercutio's brother, or his relationships, is entirely 100% not Shakespeare.
Doesn't mean I wouldn't want to go see it, though ;). Maybe it's a guy thing? The whole young adult high school romance vibe that Rosaline gives off doesn't do it for me, but having grown up with just a single older brother myself, I think I'd like to see a "behind the scenes" story of Mercutio and his brother, like Rosencrantz and Guildenstern.
I wonder if you can buy the script?