(Orestes): "But why didn't you drive her in flight while she was alive?"
(Chorus of Furies): "She was no blood-kin of the man she killed."
(Orestes): "And am I blood-kin of my mother?"
(Chorus of Furies): "How else did she nurture you in her womb, you foul murderer? Do you disavow a mother's blood, your nearest and dearest?"
(Orestes) (turning to APOLLO): "Now is the time for your evidence, Apollo, to set out on my behalf whether I killed her justly. I shall not deny I did it, as it is the fact; but you are now to give your judgment whether in your opinion this blood seems justly shed or not, so I may tell them here." (604-613)
It also seems safe to say that Orestes is being pretty nonsensical here. Even if you disavow a relationship, that just means disavowing the social ties of the relationship, right? Just because you swear to something doesn't mean that you can take away a blood relation, can you?