A line-up? God no, we really didn't want one...so didn't. My parents were paying and didn't mind either, the venue said they are very unusual now.
BUT: We were having quite a small wedding, (well, 60 people to the day)and my parents knew all bar about 3 couples...all of whom they met at the post church, pre-dinner drinks as they were all wandering and chatting. We did go from table to table, but this was not really necessary as we had spoken to everyone there before the meal, either outside the church or outside over drinks. We did the speeches at the beginning of the meal too which meant there was a nice lull during which we could wander round tables and chat.
Don't get me started on how rude some of our guests were though, i.e.: leaving after the starter as they had commitments later that day. Fine, but at least tell us you're going so we can say goodbye, or preferably tell us in advance so that we don't end up paying £70 odd per head for a starter and a glass of bubbly!
OP, I think you are BU and YANBU at the same time. Yes, we walked the tables but we had plenty of time, not many tables and an easy layout. It doesn't sound like they had any of these things. If you are close to your nephew (?) you could have gone and spoken to them yourself. The issue here is not the Bride nor her family, you could have spoken to them at any point but waited behind on some very old etiquette and then complained later. If your sister had been truly upset she could have said something at the time to them allowing them to solve the problem, which would have been the fair thing to do to avoid this resentful slating after. The groom could also have said something about the seating plan if he was worried, so presumeably he wasn't. We had friends (bar the noisiest!) nearer the front than some family, because we see them more and are closer to them. Nobody minded, and I come from a very traditional family.
Phew, rather long, sorry!