the question is not if it's possible or not, it could be that BR is a college entirely populated by brothers and half-brothers, but if it's the case to keep on using the same stratagem to create relationships between the different characters without having to explain anything, it seems to me a very lazy way to go.
Melanie's role is completely superfluous for now, Chad could leave Sage without any external push, why would he keep wanting to keep her tied to him?
in such a microcosm it's normal that the legend of aunt Jill has survived for so long, she's rich, her sister died, who can she be if not MC's aunt? and Nora is Josy's mother, and Tybalt is Sage's half brother etc..
staying with the story told: the DIKs and HOTs know that MC had a problem with Troy, it doesn't seem credible to me that none of them know he's Melanie's brother and tell MC, even if only to prevent him from beating him up... ok that in BADIk the surname is the most secret information in the world...