If you have access to a therapist or counselor, I would strongly encourage you to talk to them about it. You mentioned other things having happened in your family that makes you hesitant to bring it up with her, so it would be better if you could sit down and talk with someone confidentially and lay out the whole story and all its complexities. The therapist could help soothe your anxieties and fears, and also help you come up with a thoughtful plan of action as to maybe ask for her forgiveness (if that's something that is deemed suitable).
It sounds like you are destroying yourself with the guilt and pain of your actions... I think asking for your sister's forgiveness is going to help, and forgiving YOURSELF is going to help most of all. The most important thing here is getting yourself to a point where you can relieve yourself of the guilt and pain. And honestly, it really shows how sincere and good you are inside by how deeply your actions are grieving you right now. But you can't go back and change anything... you can only learn from past mistakes and then move on from there. It'll also help to check in with your sister if possible to ask for her forgiveness and see if it caused her any harm or trauma. If it did, seeing your shame and guilt and hearing you ask for forgiveness will certainly help her in the healing process. And you seeing her heal with help you in turn. And there's always a chance she didn't remember at all or wasn't bothered by it at all. So that could be freeing as well.
So my suggestion is, if you decide to actually talk to her without seeing a counselor first, I'd say to get her alone and ask her if she remembers when she was X years old and you guys used to play around sometimes. Gauge her reaction. Depending on what she says, you could then say do you remember anything bad happening? She what she says. If she remembers, apologize and ask if she could forgive you, and if it has negatively affected her in any way. If she doesn't remember, you could say well I touched you inappropriately a few times, and I feel really bad about it and just wanted to make sure it hasn't affected you. I'm sure she'll appreciate your openness and honesty in coming forward even if she didn't remember.
Best of luck, and don't be so hard on yourself.