Starting at the beginning, I know my wedding is over a year away so I'm not completely freaked out yet about which guests are on the list, who is and isn't coming, etc. I have set up a rough working draft of the guest list though just to get an idea about how many people we're dealing with and also have one place to keep track of addresses. It mainly consists of my Christmas card list, my graduation announcement list from college (minus the deaths that have happened since then) and my FMIL's list of family and friends she would like included. This ensures both our family lists and our friends are included in the numbers we're playing with.
So my younger sister (5 years younger) would have no one her age there and only would know family (the vast majority of which are senior citizens). She's never been in a serious relationship but I gave her the go-ahead to approach her best friend, who is something like an honorary younger sister to me, if she might like to come so my sister would have someone her own age to talk to. My brother is only 2 years younger than me and not only knows all my friends from HS, he was actually friends with a lot of them and was invited to some of the same parties I was. He somehow assumed it was ok to ask his 2 best buddies to come with him to the wedding...and I only found out because my sister mentioned it in passing.
My FI is not the biggest fan of my brother in general, for a lot of legitimate reasons. So when he found out about this, he flipped his top even more than I did. My plan is to give my brother a +1 on his invite, since he is family and over 18 (plus, if we're all lucky he could be dating someone by then). I figure this way I'm not playing favorites between my siblings and I only have to deal with, at most, one of his annoying friends. This would mean I'm technically un-inviting someone my brother "invited" behind my back more than a year before my wedding. I was just making sure this would be considered good in terms of etiquette.