Wiegraf was Miluda's brother, you fight her twice and are force to kill her the second time. He's quite easy if you're prepared to cheese the fight with auto-potion and yell spam, he's damn near impossible otherwise.
Meliadoul was the brother of Izlude (the pain in the ass with jump that you fight at Orbonne Monastery) and the daughter of Vormav, who gets possessed by Hashmal, who is essentially the leader of the Lucavi and trying to resurrect Altima. Izlude is killed by Hashmal when he slaughters everyone at Riovanes, Meliadoul doesn't know anything about all the demon shit and blames Ramza for her brother's death, she confronts you, escapes, follows you to Limberry Castle, sees Elmdore turn into Zalera and admit that Vormav is also possessed, so she joins you.
God the story of Tactics is so amazing. As much as I love FF4, I think Tactics is probably the best story Square ever wrote. All of Ivalice is amazing, FF12 could have been fantastic if they left out Vaan and Penelo, gave a lot more story time to Ashe and a bit more to Balthier, and added an actual end boss.