If the noble faction were in control of the government, it is quite likely that they would demand that she divorce Guy before inheriting the throne. It is not very likely that Balian of Ibelin would have divorced Maria Comnena in order to marry Sibylla, as Balian and Maria had a very happy marriage.
The native nobles did demand that she divorce Guy as the condition upon which she became Queen. She said that she would divorce Guy, then, when she was being crowned, she put another crown on Guy's head, crowning him as King, and making the native nobles' demands void. This caused a big rift within the Crusader States as many of the native nobles refused to swear alliegance to Guy.
Baldwin of Ibelin or another native baron would have been a better suitor. This situation is confused because of the 2 factions which were struggling for control of the central government. The native faction under Raymond of Tripoli and the "westerners" such as Amalric of Lusginan, Reynald de Chatillion, Joscelin III of Courtenay, and others. If one faction gets Sibylla to divorce Guy, then the other would have protested and the kingdom would have been thrown into a suicidal civil war.
The "westerners" did win. With Guy crowned as King of Jerusalem, their guy was now in the driver's seat. Raymond of Tripoli native faction actually tried to start a civil war by getting Sybilla's younger sister', Isabella, and her husband, Humphrey, to declare themselves as the King and Queen of Jerusalem. Humphrey was weak, and very loyal to Guy, and so swore alliegence to Guy rather than start the civil war the native faction wanted.
As I've stated before though, going over these very complex medieval politics is fun, but it doesn't really accomplish anything along the lines of, how to get the Outremer to survive. Without Aleppo and Damascus the Crusaders' goose was cooked no matter how rational, intelligent or lucky they are. If Saladin can't get the job done (I'm of the opinion he would have barring death), then the Mamelukes who take over a few generations later will. In my opinion Enduring Outremer what-ifs fall into three levels of plausibility: First Crusade and Post-First Crusade any what-ifs are based on intelligence and luck, Second Crusade what ifs are based on lots of intelligence and lots luck, and Post-Second Crusade what-ifs are based on ASBs. This is the thread falls into the last category.
So how about some First Crusade What-ifs?