Wow, it must have taken a lot of courage to come out and speak out for yourself when it came to your dad's encarceration. It almost angers me to read that your mother made excuses to cover up what happened when you were young. My mother did the same thing. Do you ever think that feeling resentment is there for a reason. I do. It sounds as if your sister is in denial of that and is affraid of dealing directly with the issue. Have you considered confronting your mother once again (when you feel ready). I am a firm believer in that, the issue's aren't gone if your still angry about them. There is more than one kind of abuse. You have direct abuse, and you have the abuse that was done to you by the neglect from the people that were supposed to protect you. Excuses can be made, the abuse can be minimized, and false stories can be told. However, it sounds as if you know what's really going on and you're pissed. That is definitely a good thing. I myself have been through similar issues and I wonder if you have ever looked into any literature around how to confront someone for issues like these. Maybe speaking to a counselor or reading up on confrontations in some books involving sexual abuse. They don't only talk about confronting the abusers but they talk about confronting third parties that are somewhat responsible for the abuse even taking place and the covering up of it. If and when you confront, remember, the person who remains calm is in control of the conversation. If you talk to her, it is likely that she'll get defensive and lash out in different ways so it's important to keep a level head and remain focused. This may take you several times to get it all out but It can be extremely beneficial to you and your relationship with your mom.
Truth is the very reason we strive to live. It surrounds and resides within us. Accepting the truths we already know and seeking out those we do not is a direct path to inner balance and joy. For life is not a means to an end, but a journey. Life comes and goes but the truth will always live on.