Due to increasing Alzheimer's symptoms, my sister's family has decided not to bring her to her son's wedding. She is feeling unwanted; was it the right decision?

Yes, this was the right decision. There is a point where there are just some things that we can no longer do. That does not mean the rest of the world has to stop to accommodate us. Your sister would not have enjoyed this trip and whoever was in charge of her even less. At worst she could have been disruptive to the celebration or required emergency medical care, ruining the entire trip.

My father was very popular with his nieces and nephews. There was a wedding several states away that I planned to attend with my daughter. At the time my father was still living on his own with help from me. I was concerned I would be pressured by relatives to bring him and decided if that was the case I would not attend. I wanted to be a guest, not a caregiver. I had visions of him finding an excuse to go to the ER while we were there.

we took my mom with dementia to an out of state wedding once and after the ceremony she kept asking everyone to take her home, and had a bit of a meltdown until someone did agree to leave the wedding (reception hadn't even started yet) and take her back to her hotel, and that person had to stay with her of course because she couldn't be alone, so missed all of the fun. most of us were then bummed out after that scene and didn't enjoy the reception and left early, so it was ruined for everyone basically. it is the right decision not to take her, and as some others have said perhaps telling her that they are going to have a reception at home might appease her, even going so far as to tell her that is the actual wedding so she doesn't know she's not going. i imagine if she has moderate dementia she will forget she isn't invited, no?