When the mc says to Derek, "You know exactly why" I'm sure it's all about the crabs, except it's not likely you'll get crabs from kissing someone, but in Arieth's case, she's already "huba huba"ing up for the MC, so you don't really want to egg her on.
I don't know if it's cheap and lazy to do that. Considering the lengths DPC went to set the Chad/Troy/Mel triangle up, the various subtle and not so subtle scenes:
- The information that Troy is an ex-Jock for unknown reasons,
- Mel or Sarah being an obvious candidate as the "side bitch" but then being on the treadmill during Chad's gym phone sex,
- This vague discussion between Mel and Sarah when Sage is kissing the mc on the lawn,
- The blackmail to Chad (that I believe may have come from Mel to force Chad's hand),
- The numerous references to Mel & Sarah liking gossip, but their seriousness when it comes to Chad's relationship,
- The flashback scene showing Chad disinterested in puppy dog Sage,
- The pre-breakup scene with Chad and Sage when she confronts his lack of interest,
- The exasperation in Chad's face during the breakup,
- Mel's unhappy reaction to the breakup scene (even though it's what she wanted),
- The Jocks chatting about Chad's pleasant demeanour since he's left he jocks and his role as president,
- Mel (and Sarah's) continuing support for Chad after the Sage breakup.
All of the above was to build the Chad, Troy, Mel and Sarah story, and it works if Mel is related to Troy and working towards what she wants for him.
She could also be an ex-girlfriend of Troy's who realised his secret and wants to help him with it, or even just a good friend, but being a family member makes more sense considering Mel doesn't come across as a girl that gives too much of a shit about anything, but family is different.
If Mel is Troy's sister, that's not a ridiculous coincidence, it's a requirement for this story arc to work, it might not have happened if she wasn't involved.
That's very different to Tommy and Josy being step-siblings. Almost nothing in the story hinge on that, so it is just a simple fact in the game. Same goes with Tybalt being Jade's son.
With Maya and Derek, we have a lot of their past presented to us. Their relationship with their father, their trips together (including Josy), and their care towards each other. It's certainly not a lazy plot contrivance.
Your comment earlier, that it would have been better if Maya hosted the mc without the prompting of her brother, I don't agree there.
She knew him for one day, sure they hit it off, but the mc would never have asked her if he could stay with her (he wouldn't have known she had the room). Plus she loved her extra room, it wouldn't make any sense her offering it up to a guy she just met.
Derek had to call on a favour to talk her doing it. Given everything we know about her, if she just outright found out the mc was homeless and offered her space to him, it would have been criticised as unrealistic.
To me, none of the above is lazy, it's actually a well construed sequences of life events. It works because it is what it is. Take one element out and it doesn't work.