The Melanie/Troy theory is based on some (albeit vague) evidence:
- She's aware of Chad's relationship even before Chad has broken up with Sage.
- It's clear Melanie has an invested interest in Chad breaking up with Sage (Mel's discussion with Sarah when Sage and the mc are kissing on the lawn).
- The way Melanie reacts during the actual breakup fight, she's very upset (for Sage) but it's obvious, (from the previous discussions with Sarah) that ultimately it's what she wants.
So why would she behave like this? There's heaps of possibilities, and being related to Chad or Troy is one of them. I pick Troy 'cause she looks more like a Troy sister than a Chad sister, but that's pretty arbitrary. Sure, she could have been a childhood friend, or an ex-girlfriend, but if Chad has managed to hide his homosexuality from Sage for the years they have been together, it's unlikely that too many people would know about it, family could be one exception (i.e. family of the side bitch).
Compare all that to: Jill has a dead sister called Lana, she must be related to the mc's dead mom called Lynette because... they're both rich... dead... bitches... There's fuck all reasoning there. That's the difference between the Lana/Lynette theory and the Troy/Melanie theory.
Still doesn't mean either of them are right or wrong, but one is founded in some form of reasoning, the other is just random.