@shanefolan29 I am sorry you have had some very harsh responses from people who have no idea what your sister did to you (and there is no need for you to tell us).
I completely understand your need to be away from your sister. I this situation I feel it may be worse for you to go and perhaps have your sister try and talk to you or your mum or other family members trying to get you two 'together'.
In your shoes I'd speak to your brother. i assume he knows what your sister has done and has chosen to try and be in touch with you both. Explain you can't be around your sister but want to celebrate with your brother and his new wife.
This could mean you attending the church or legal ceremony and sitting away from your family (they do not even need to know you will be there, to be fair maybe your brother doesn't even need to know if it is in a church as I think the service is open to any to attend in a church - check that if you want!).
or
Ask if you can meet your brother and new wife soon after the wedding to look a photos and hear about their day.
If your sister's children are under 18 (or at the very least 13) it is not appropriate for you to try and maintain a relationship with them, since contact would be via your sister. However, once they are adults they may or may not want to see you and you may or may not want to see them.
Life is way too short to be around people who make you feel really uncomfortable or unhappy. You know this and have made your choice.
This is not about other family members, it is about you, it is your life.
Your brother's wedding day will not be spoiled by your not being there and the fact your family have not decided to be in contact with everyone suggests they do not think what your sister did was so serious (or do they simply not believe you?) Either way, you must do what makes sense for you in your life.
Good luck, and I hope you find peace and that you and find a way to celebrate your brother's wedding if you wish to.