I've pretty much RTFT and my head is wobbling.
So, as I understand it, OP did 'something' (not illegal, not immoral) for X reason. His sister attributed his motive as Y (something 'sinister') and started talking smack about 'it' (and her interpretation of 'it') to other family members(?). OP got all het up, told the parents all about it and then left the area with no further contact with sister. And whatever 'it' is, it is not sexual assault/rape/child sexual abuse. Also, whatever 'it' is, OP has 'hard proof' that his motives were entirely above board.
I think part of the problem with other family members is that 1-you left the area immediately after dropping the bomb, and 2-you are trying to make others 'take sides'.
1- some people might interpret your leaving as 'running away from guilt'. Not saying you are in any way, just the way some people might look at it.
2-trying to get people to take sides in a dispute when they don't want to usually ends up with them siding against you. Especially if the other party isn't making the same demand.
I suggest you call your brother, explain clearly what happened and that although you do not want any rapprochement with your sister, you would never cause a 'scene' during his special day but would quietly leave if approached. Then leave it up to him. If he still wants you there and you can agree to 'terms' (i.e. not being seated next or near to sister, no trying to force contact etc), then you need to speak to your parents, explain that you will attend but that if they attempt to make you interact with sister, or if sister approaches you, you will leave.
Also ask brother if you can bring someone to run interference for you. A friend or sig other who can intervene and get you away from an awkward encounter.
Oh, and plan to stay at a hotel or with friends. You need a 'bolthole' where you can retreat if needed. Do NOT stay with your parents or other relatives. You do not have the right to demand that they bar their homes to your sister because you are staying there.
We've only had one rather stupid, minor squabble between two uncles. The family line was to invite both of them and refuse to confirm whether the other was attending. Each was free to then accept or decline of their own volition.