It is easy to get Edgar and Edmund confused because of the similarity of their names. The answer by michellesherman is of course correct. It was Edmund who was loved by the two sisters. The fact that both Regan and Goneril were evidently so madly in love with him suggests that Edmund would have been played by a handsome and graceful actor. Edmund's motivation to become such a cunning villain is at least partially attributable to the fact that he knows he is exceptionally good-looking and gifted in so many ways. He feels he should be a nobleman with a title and property, that fate played a dirty trick on him by creating him with so many superior qualities without giving him any rank or money; and he must secretly hate his own father for conceiving him but not giving him the means for advancement in society. Gloucester jokingly calls him a "bastard" and a "whoreson" when he introduces him to Kent in the first act of the play. Gloucester might think it funny, and he might be proud of the fact that he was able to sire such a "proper" son (as Kent calls him); but Edmund doesn't think it is a bit funny, and he accumulates a lot of secret resentment and anger which he vents on his own brother and father. Edmund is like Heathcliff in Emily Bronte's Wuthering Heights. He appeals to some women partly because of his smoldering emotions and his bold, reckless character. Edmund is quite a different villain than Richard, the duke of Gloucester in Shakespeare's Richard III, who says of himself that he has decided to become a first-class villain because he is so ugly and deformed that dogs bark at him when he halts by them. Shakespeare was right on target it perceiving that people's characters and behavior are shaped by their perceptions of themselves.
It is Edmund. He is referring to Regan and Goneril. The passage you're asking about is
Yet Edmund was belov'd:
The one the other poison'd for my sake,
And after slew herself.