Do you think that Gerold really killed Cerelle?
Tybolt succeeded him as Lord of Casterly Rock, only to perish himself two years later under suspicious circumstances. A young man in his prime, Lord Tybolt left no heir of the body save for a daughter, Cerelle, three years of age, whose reign as Lady of Casterly Rock proved cruelly short. In less than a year, she too was dead, whereupon the Rock and the westerlands and all the wealth and power of House Lannister passed to her uncle, Gerold, the late Lord Tybolt’s younger brother.
A genial man, known to be exceedingly clever, Gerold had served as regent for his young niece, but the suddenness of her death at such a tender age set tongues to wagging, and it was whispered widely in the west that both Lady Cerelle and Tybolt had died at his hands.
No man now living can say with certainly whether there was any truth to these whispers, for Gerold Lannister soon proved himself to be an exceptionally shrewd, able, and fair-minded lord, greatly increasing the wealth of House Lannister, the power of Casterly Rock, and the trade at Lannisport.
It’s possible he killed her, but my immediate reaction is no. He already had all the power as her regent, and if he really wanted to become Lord of Casterly Rock, there was a legal way to do that without kinslaying. Uncle/niece marriages aren’t strictly prohibited by the Faith, though consummation would have to wait at least 10-15 years.
I tend to think we’re meant to draw a parallel with Tyrion here, who was also wrongfully blamed for the death of his brother’s child. (“I overheard some, m'lord,” said Shae, “[…] That was when he told me how he meant to be king.”) Gerold also liked books and didn’t get along with Tywin.
I also don’t think Rohanne would fall in love with a baby-killer, but that’s just my interpretation.