in death, they thought kathleen peterson and elizabeth ratliff could have been twins. >> i'm just thinkingase is getting a whole lot better. >> reporter: kathleen's sister candace thought that peterson had killed both women. >> i have a better chance of being struck by lightning than finding two people who i intimately know at the bottom of a staircase. >> reporter: but to martha and margaret, the whole thing seemed absurd. the fact that michael was being accused of killing kathleen, the woman they called mom, was bizarre enough. but now, their birth mom, too? what would their father have gained by killing elizabeth ratliff? >> he would've gotten two screaming little ragamuffin kids out of it and that's it? like, there's nothing -- there's no reason for it. >> reporter: for the investigators in north carolina, though, the death in germany became a strong building block in their circumstantial case for murder. and what's more -- detectives learned that michael peterson had a secret life. secrets -- tawdry ones -- were about to spill out in the durham courthouse. >>> coming up -- enter brad,