Unless you have not told us everything (like penetrating her, or whatever), then I don't see much to worry about. You are right that she might remember stuff at that age, but it's unlikely that she'd interpret it as strange.

If I was you, I would worry less about your sister, and more about your father: did you sort matters about the abuse he inflicted on you? And the previous abuse you mentioned?

Also, I don't see the need to speak about your sister issue with your mother, but I would tell her about what happened with your father. Is she already aware?