And wouldn't Harun now be the cousin of the Grand Caliph, and if the Grand Caliph dies before having an heir, then his father will be the Grand Caliph, and then Harun is the heir? Which means he is currently third in line to be the Grand Caliph?
*If* you are the eldest son it would appear so?
OK! Time to break out the Grand Caliph's genealogy! Not to be confused with genieology
NOTE: There's an error on this table.
Makin al-Mutttsysn should be
Makin al-Mutrattab.
So Makin is Prince Tannous' son. In the books, he is described as a 10th-level fighter, former adventurer companion of Grand Caliph Khalil (his uncle) when the two used to sneak from the palace, and in his adult life he was appointed Chancellor of Huzuz where he proved deft and effective at city politics. This description – despite being lower level than Husam – suggests he is the "responsible older brother." Well, since he's human and focused on city life, he could very well be Husam's older half-brother?
Interestingly, the books describe Prince Cheddah (a "buffoon") as 2nd-in-line after Prince Tannous...so that says something about how succession works in Zakhara. It would pass to the Grand Caliph's son, but if he has no son (and won't recognize a daughter as heir), then it passes to his eldest brother, and then to his youngest brother – succession appears to only pass to his brother's sons (his nephews) in the event that his brothers die first.
Going with this version would indeed make Harun a potential heir...albeit he's pretty far down the totem pole after his father Prince Tannous, his uncle Prince Cheddah, and his older brother Makin.
Awesome. Thanks. Next dumb question. How does one hide being a water genasi or are they more common than I'm imagining them to be?
No dumb questions

Well, I don't know about genasi since those weren't invented until after Al-Qadim came out, but jann and other genies are ALL OVER Zakahara, with several serving in the Grand Caliph's court. And Zakhara is a multi-racial society. So I don't think it's the kind of thing that would elicit tremendous shock unless you were among a particularly superstitious or remote peoples.