In my family, we have a policy on not keeping secrets - because of a similar situation to that which you've hinted at, OP.
My mother unburdened herself to me, when I was a young teenager, about her stepfather having sexually abused her. Even as she was telling me the sordid details, I chose not to burden her with the fact that her brother-in-law had sexually abused me as a young child (although my grandmother, his mother, knew and did her best to protect me - without telling anyone else about it). I regret not having done so, now. However, the fact remains that my mother's claims about my grandfather were horrific enough that I never felt comfortable around him, again. To this day, I don't know if she told me the truth that day (and there is a distinct possibility that she wasn't being honest), but it placed a niggle of doubt in my mind. It also made me question why, if he had abused her as a young child, she had left me in his sole care for weeks at a time when I was growing up?
Her unburdening resulted in my relationship with my grandfather being permanently destroyed (and I idolised him as a child) because I found myself sifting through previously fond memories and wondering if there was something dodgy about his behaviour towards me (I know that he didn't abuse me, but for a while, I did find myself wondering)... but it also severed another strand of my relationship with her. Why? Because I knew, even as a young child, that I would never leave my children alone with my father's brother, for fear of his abusing them as he abused me. Consequently, when I became a mother, I kept him away from my family. He met my DD once, at a close family member's funeral, when she was a toddler, was never left in a room alone with her, and that was the last time that we saw him. My children know that I was abused by someone as a child, but they don't need to know who by (he is dead). It's enough that they are aware of the fact that this is why we don't keep secrets in our family.
I doubt I will ever forgive my mother for unburdening herself on me. I spent every single summer holiday with him as my primary carer and he was the only family member brave enough to stand up to my mother on my behalf for years. All those memories of my childhood have been tainted. Remain tainted. Even now, when I think back to sitting upon his knee as he read to me... I wonder if there was something untoward in the way he held me securely upon his lap.
If your grandfather has been dead for 20 years, and your mother's unburdening of herself to you has caused you to feel similarly to the way I do...? Don't tell your sister. Tell your friends, tell your therapist, tell your partner (if you have one). But telling your sister will only cause her to feel the same confusion and pain that you do right now - and I don't think that you want that for her, which is why you haven't already told her your mother's secret.
And, as a previous poster asked... why did your mother unburden herself to you and not to your sister? That might be worth discussing with your therapist in and of itself. Does your mother have a habit of trying to cause fissures in your relationship with your sibling(s)? Might her unburdening of herself to you not have been the truth?
