So I just got engaged a few weeks ago (wee!) 
FI and I have been dating about 3 years. I get along with his family for the most part. 
He's an only child, only grandchild, etc, and his mom is very protective in that only-child way, and i'm anticipating and willing to accomidate how much of his extended family may want to attend. It's early in the process, but after reading a LOT of FMIL drama on here, I think I may come out lucky on that.
Naw, I think the trouble's going to be with MY family. 
Both my (divorced and re-married) parents are GREAT actually. The trouble comes from 1. My brother, and 2. my aunt, uncle, and their two kids (about my age). My brother... how do I put this... y'know how older brothers can sometimes be just needlessly cruel and abusive and bratty as kids, and pick on you and start trouble and just be an all around %$#%$#^? Well my brother is almost 30 and still in FULL 
@#$%#!#$ mode. I don't know how to describe it other than he's an incredible narcissistic, rude, insulting person who thinks his worth is MILES above anyone else. He's simply always been that way. There's no rhyme or reason to it, and as a result we do not interact much at all. 
Not even a week ago I made a comment on my FB page that I was looking forward to my new sibling (my step-mom and dads) being born so I could be an older sibling and torment them so i'd get to know what it was like. TOTALLY in a joking, playful manner. My brother saw it, and left (I KID YOU NOT) a 12 paragraph long diatribe on every tiny little thing, real and imagined, that I ever did to him as a child (Did I mention he's in law school and likes to do this a lot? like, starting arguements where there are none). I say imagined because he said a bunch of things I supposedly did to him that were flat out lies.
Mostly, I was mad because our family, grandparents, and MY clients (i'm an LMT) see that page, and it was childish and disrespectful and downright BULLS$#% for someone who's almost 30 who has a wife, kid, and one on the way. So I deleted it, wrote him a private message telling him my page doubles as a professional page my clients see, and if he has a problem to message or call me privately, and not try to be a law school attention whore (not what I said, exactly) in public, and blocked him completely. As a side note, he's actually done the same thing to my mother AND grandmother at some point. So I just blocked him and forgot about it, because sadly it's nothing new.
A few days after, my FI gets a message on HIS page from my brother, about how he needs to look into my past and make sure I have everything "taken care of with *ex-husbands name* before he gets stuck with me" 
WHAT. THE. %#
@^. 
Backstory, i'm 26, and was briefly married when I was 19 in a young and dumb phase. Its been "taken care of" for 6 years, and of course FI knows about it. 
At this point, I don't want anything to do with him. I've had very little contact with him as it is, but now I don't want to see him at all. ever. 
But here's where it gets hairy. I like his wife, we were friends before they started dating, and my nephew is awesome, as little as I see him. I spoke to my dad about this and he says he knows what i'm talking about, but that he keeps the peace because he wants a relationship with his grandkids.
My gut tells me that I want NOTHING to do with someone like my brother. Our relationship has been crap ever since I was born, so I don't really feel like i'm losing anything. Whats bothering me is that if I -don't- invite him, all I will get at my wedding is "Where's your brother?" "omg, you didnt invite your own BROTHER??" etc etc. 
Same goes for my aunt/uncle/cousin situation. Long story short, half of my extended family on my dad's side is one religion, and half isn't and they all live in the same small town, literally down the same street. My grandmother who is very religious, is just the sweetest, perfect "granma" type lady in the whole world. My aunt/uncle and his family, are the opposite. They're judgemental, they spread rumors and talk trash and cause drama, to the point that I've only been in their presence when I accidently see them in a store or happen to be visiting my grandmother when they are around and its been that way for at least 8 years. Because I am not their religion, they tell everyone I am a bad influence and that my mom and dad were horrible parent's etc etc. I'd be fine with ignoring them if they don't like me, but its the passive aggressive gossiping even when i'm minding my own business that pisses me off.  So by leaving them out of the wedding (honestly I have NO relationship with these people), I can only imagine the gossipy crap that would probably ensue. 
Like with the situation with my brother, my gut tells me I do not want to have these people talking crap in the back of my mind on my wedding day. Its stressful enough, and i'm on a mission to make this as stress free a process as possible. But at the same time i'm worrying about the backlash with all these relatives in the same town, and how i'm supposed to deal with my brother when I don't have any problem with his wife and wouldn't mind HER being there, but I can't just invite her without coming off like i'm being spiteful or something, y'know? 
Blah. sorry this is so long.