a private e-mail gone wrong?

ok, i sent a private e-mail to a friend of mine and i donno what happened, but the e-mail somehow got sent to my sister. i think while i was clicking, in my address book, on the person i wanted it to be sent to i accidentally clicked on my sister e-mail address. i've been trying to figure out what her password is so i can go into her e-mail and delete it before she reads it, but i cannot figure out her password. what should i do? i REALLY don't want her to see this e-mail!