Sometimes believing a half truth is better than believing a whole lie. Meliadoul had once believed that Ramza had been the one to kill her brother. When she confronted Ramza to accuse him of murder, he had put the blame of Isilud's death on the hands of their father. She had thought it absurd to think that Folmarv was capable of filicide.
Meliadoul was confronted with the truth when Zalera, the Lucavi demon that had possessed the body of Marquis Elmdore, told her that her father had willingly let himself be turned into a Lucavi. She could no longer believe Ramza had been the one to slain her beloved brother. Folmarv had his form taken by a Lucavi, had killed his own son. She knew this to be true, but she couldn't believe her own father would willingly let himself be turned. Meliadoul let herself believe Folmarv was murdered by the Lucavi known as Hashmal, then had his form stolen. To not believe in this would hurt far to gravely.
She would slay Hashmal to avenge Isilud, and maybe even her father too.