Motivation + Third Daughter?

I have a question about the dialogue during the confrontation between Electra and Clytemnestra.

Electra says:

"And then, as you said, he let Iphigenia be killed.
Why did you throw me and my brother out of our country?
Why did you give a stranger our inheritance?
And why did they kill my sister?"

From what I know of Greek mythology, Electra was suppose to have two sisters: Iphigenia and Chrysothemis. When she says "and why did they kill my sister" does she honestly not understand why Iphigenia died and why Clytemnestra was vengeful, or is she referring to the death of Chrysothemis?

I am not familiar with any stories about this third daughter... was she also sacrificed or killed by her father, or stepfather?