Ginga Densetsu Weed
They and thair siblings were adopted by a traveler who offered their father a boar in order to take them with him so he could take care of them and make an end to the abuse. Apparently, the traveler took good care of them but they left him when they heard their mother was ill.
The three brothers returned to their home, only to find their mother's skeleton. Bitter towards their father, they were sure he killed her and wanted to kill him in order to make him pay for all he did. After long time of searching they found him and trapped him infront of a cliff.
Then, the two brothers committed a suicidal attack by pushing themselves and their father off the cliff. They did this because a killing of one's own father was a horrible crime and they didn't want to live after committing it.