exoticfruits your day sounds lovely but you've said several times, going into the other room to watch the entertainer was optional.
That's not the case here according to the OP, her sister has said that all the children must go to the entertainer and that it is not debatable. Very different to your arrangements. That's what the OP and many others have a problem with.
I agree with the people who have said an entertainer is very different to a babysitter. He or she will not be stopping the act to help or comfort children or follow them about if they leave the room.
Which may not be through a door which adjoins the one the OP is eating in, so potentially young children could be wandering about the venue unsupervised and without their parents having any idea they were not still with the entertainer in the other room.
DS is just three and would probably be quite happy without me, but if they are eating while they watch the entertainer his is still prone to spilling juice or dropping food, if he hurts himself he still cries and wants a hug and he is going through a bit of an accident prone bumped head stage, and if he were dressed up for a wedding he might struggle to get his trousers down for the toilet (which he wouldn't be able to find on his own in a strange hotel/wedding venue and would need someone to take him.
So for those reasons, I wouldn't be happy about leaving him alone and would either want to stay with him or I would be up and down all through the meal and speeches to make sure he was alright and not in need of anything.
The children at our wedding stayed with their parents and had their meal with the rest of the wedding party. To counteract the boredom we gave every child who was invited a gift bag of toys, books and colouring things to keep them occupied and they all had balloons on their chairs. They weren't any trouble and even the bit on the wedding video when you can clearly hear my nephew saying "Look! It's got a boot and it opens!" as he discovers his toy car if funny rather than annoying.
I'd have preferred that over a video filled with parents getting up and down to check on the children or the children running in and out because they needed to go to the toilet or had spilled something or were upset or scared or whatever. OP I think that will be a flaw in the plan for your sister, it's not going to be out of sight and out of mind for parents or children.