Can anyone help me with this problem?

I have this family dilemma that needs solving. I have 2 older sisters. I'm 20, Marilyn is 26 and Sarah is 32. My parents divorced when I was 3. They were married for 13 years. I live with my mom. My dad died in Colorado 4 years ago. Now here's the dilemma: My sister Marilyn has ADHD. My mom has bipolar, schizophrenia, and manic- depression. Marilyn was known to make up lots of stories in her youth. She still does. My mom and dad were in an abusive relationship. They would hit each other and my sisters. After my parents divorced, my mom had a nervous breakdown and she couldn't take care of my sisters anymore. I stayed with my mom. Marilyn was in and out of foster homes and she's been in the Training School twice. She's also been in trouble with DCYF. When Marilyn was 16, she told her case manager that my dad had molested her when she was 8- 9 years old. My dad was arrested and he had to take a polygraph "lie detector" test. The first time he took it, he failed. It said that he had, in fact, molested my sister. He took it a second time. He ped and it said that he didn't molest Marilyn. My mom never noticed anything when they were living together. I never had the chance to ask him directly, and I don't trust my sister since she's always making up stories to cover herself. So I have to put my trust in the polygraph. But which polygraph should I believe? The one that ped or the one that failed? And I thought that those things were foolproof. And as sick as it sounds, wouldn't my sister Sarah have been a "better" choice? Not only was she 13-14 at the time, she's also adopted. Sarah is my half sister on my mother's side, so that means my dad adopted her. And that usually fits the profile of a parental molester. I don't know who or what to believe. This has been plagueging me for so long and I just want a straight factfilled answer. So, please give me your opinions on this matter and put my thoughts to rest. Thank you.