Weddingbee boards

Well, first, they are 12-19, not 3-4 so I don't think you have to worry about them acting up at the wedding.  While they may let loose a little bit in their own home, I'm sure by that age they know how to act in public.

Second, I'm not clear on who the nephew belongs to- is he the son of brother 1 or brother 2?  And where do these girls live?  Do they primary live with brother 2 and gf, or with their father?  Assuming that nephew is the son of brother 2 and the girls live with brother 2 and gf, then I think if you are inviting one child of the household (nephew) then you must invite all children of the household.  It would be really mean to say, "well brother 2, I know you consider these girls to be your children, but we don't, and only your biological child is invited." which is essentially what you would be saying if you refused to invite them. There is no amount of budget-saving that would make sending that sort of message ok, and doing so would probably severely damage your relationship with brother 2.

Now, if the nephew is NOT the son of brother 2, but is part of Brother 1's family, then I think you have more leeway. In that case you could say that the nephew and nieces were invited because they were part of the wedding, but no one else's children were invited.  I would not mention to him that you feel close to your nieces and nephew and not to his kids, because even if its true he's likely to take it the wrong way.  But I think this only works if the nephew isn't the child of brother 2, you know?  Think about it this way, if you had decided to have one niece in the wedding but not the other- would you tell brother 1 that he had to leave one child at home?  That probably wouldn't have gone over well.  So that's why I think if you invite one child in the household, you should invite them all.

I really think you should take your guidance from you FI and his brother on this one.  Your FI wants them invited (and it's his family), your brother considers them his daughters, and going against this is going to cause you way more drama than its worth. 

So FI's nephew is Brother #2's son by his previous relationship?

I always like to make hard-and-fast rules and stick by them.  In my case, my rule was relatives get to bring kids if they're still living at home, and coworkers don't get to bring any kids... no exceptions.  (Not sure what the rule for close friends would have been, but I only have one friend who has a kid, and the kid is my flower girl.)  I think if I applied my rule to your situation, then I'd probably end up inviting those three girls.  If Brother #2 and GF live as a family unit with kids, it would be hard to explain to them why you're inviting the boy but not the three girls in that family unit.

I voted invite them.  You mentioned that FI's bro and his GF have been together for a few years and these girls are like daughters to him.  And while they may not be family to you yet, they are family to your FI's bro (and very well may someday be family to you).  I think it wouldn't be the greatest situation to exclude them.  It's almost like you're sending the message to the girls, "Hey, your good enough for FI's bro, but you're not good enough to come to my wedding."  It's almost like how some people say, "you invite one cousin, you invite them all."  You can't invite part of a family, and exclude the rest of that same family.  It would be different if these were adult children living elsewhere, but these are still young kids living at home.  I think it would create some serious tension later on town the road.