You say you do not want to get caught in the middle. You do not want to take sides. I could have handled it if you had told me you did not believe either your sister the accuser/victim or your dad the accused/predator who denies any wrong doing, but you do not. You are not neutral. You have chosen sides. You are saying you do not believe your dad sexually abused your sister. How is this not taking sides?
You say it may be false memories. This means you believe in false memories. Well if she is having false memories, then could you not also have them? Could your lack of remembering anything out of the ordinary going on in our house be a false memory? The mind does amazing things with stuff we do not want to admit or deal with.
I am also not sure as to your full belief in this false memory approach. If you believe it happened and she blames her dad or if you believe it never happened by anyone? If you believe someone else molested your sister and she just thinks it is your dad, then how can you remain calm about it? If this is true then there is someone out there who can do it again. It would have to be someone close to the family as outsiders were not ever in our home. If you say she is making it all up as a false memory, then you need to ask yourself why? You also need to ask yourself why she would say it was her dad if it was not? Making a claim of sexual abuse from childhood when your an adult is life altering and not easy. What do you see as her motive?
For me their is no doubt that it is not a false memory. The signs were there, I just did not push hard enough for the answer at the time and I missed and misinterpreted some of the signs. Too many facts are present that point to it all being a reality. I can guarantee you that my belief is based in the facts and reality of her life and my marriage to your dad. Many things you did not know about and will never know about. I have given you facts, but you still do not believe me. He has given you only his word, and yet you believe him.
Please do some heart, mind, emotion, and soul searching. Give some answers and support. Then ask yourself and provide answer to this: Are you sure of the side you have taken?