It is always hard when family/family are involved.
Since a wedding is a biblical ceremony I would have to say your sister was wrong in attending. Should you have said something to your Christian sister? Yes. I think so.
She didn’t necessarily need to say something to her friend about why she didn’t attend.
Tolerance does not equal participation.
My sister didn’t know what to do. She could not see herself attending, shaking hands, congratulating, etc. She discussed it with me and we both agreed that attending is condoning. The only other option would have been attending and speaking the truth at the “wedding”, but this did not seem at all appropriate.
She decided to tell her friend the truth. She told her that she just could not attend because she held fast to God’s teachings. She basically spoke the truth, in love. She did not attend, nor did she acknowledge the occasion in any way.