Lie detectors are commonly used in cases of child abuse to find out whether the perp is lying about his progress in therapy, things like that. The problem is that as legal evidence the results aren't admissible; extreme emotional stress can throw off the results, and a cold-as-ice pathological liar can fool the machine.
The other problem is that even if you find the abuser this way, it ma harm the other innocent family members to see that they were all doubted. An innocent person will of course take the position that your lack of clear memories isn't grounds for suspecting everyone.
I would take this up with a therapist and get a professional perspective. Clearly it's important that some answer be found, but this way could cause a lot of ill will among people who had nothing to do with the crime.
Nobody living can ever stop me
As I go walking my freedom highway.
Nobody living can make me turn back:
This land was made for you and me. (Woody Guthrie)