Actual:
Khal Drogo executed his brother in-law, Viserys Targaryen.
Robb Stark executed his distant relation, Rickard Karstark.
Tyrion Lannister murdered his father, Tywin Lannister. (Kin, only because GRRM has not confirmed my brilliance!)
Stannis Baratheon engaged in black magic to assassinate his brother, Renly Baratheon.
Lump used a dog to kill his brother, Bump.
Danerys Targaryen euthanized her husband, Khal Drogo.
Olenna Tyrell murdered her grandson in-law, Joffrey Baratheon.
Petyr Baelish murdered his wife, Lysa Baelish.
Ramsay Snow murdered his brother, Domeric Bolton.
Cersei Baratheon and Lancel Lannister conspired to place Robert Baratheon in harm's way.
Tywin Lannister got Walder Frey to murder Robb Stark, Tywin's grandson's brother in-law.
The black brothers went crazy at Craster's and killed each other.
(At least) One of Gregor Clegane's wives died mysteriously.
Cersei Baratheon had a number of her husbands bastards murdered.
Assumed:
Euron Greyjoy hired a Faceless Man to assassinate Balon Greyjoy.
Attempted:
Stannis Baratheon conspired to kill his nephew, Edric Storm.
Joffrey Baratheon had Mandon Moore attempt to murder his uncle, Tyrion Lannister.
Gregor Clegane tried to burn his brother, Sandor Clegane, perhaps to death.
Gregor Clegane swung a killing blow at his brother, Sandor Clegane.
Schemes still underway:
Victarion Greyjoy is plotting to murder his brother, Euron Greyjoy.
Aeron Greyjoy is plotting to murder his brother, Euron Greyjoy.
Cersei Baratheon has put a bounty upon the head of her brother, Tyrion Lannister.
Tyrion Lannister has sworn he'll kill his brother, Jaime Lannister.
Petyr Baelish will allow his stepson, Robert Arryn, to die... or mayhaps he'll just kill him.
My own theories:
Jaime Lannister murdered Aers II Baratheon.
Cersei Baratheon hatched a plan to murder Jon Snow.
Danaerys Targaryen wants to kill her siblings, Jaime Lannister and Cersei Baratheon.
Edit: Cersei has also schemed to murder her daughter in-law.