Ok so one of my brother's isn't going to be able to make the trip to my wedding but my nieces and nephew are coming. I found out that my niece (19 y/o) is planning to bring a friend to the wedding but she hasn't even asked! Our wedding is on the small side with family and close friends attending and since I knew my brother and wife weren't going to make it I figured WE could either save the money (it's a per person event) or invite guests of our choosing. Am I being unreasonable? Is it ok that people invite someone that isn't invited to replace someone that cant make it? My worry is that my nephew will want to bring a friend as well. Is this common? because a friend of mine wanted to invite her boyfriend's 2 cousins since her boyfriend couldn't make it to our engagement party! Please advice

and sorry for the wordiness!
Re: Guests inviting friends to replace guests that wont be attending
Just tell them that even though their dad isn't able to come and while that does free up one spot you aren't able to accomodate each of them to be able to bring a friend and it's not fair if one can bring someone and the other can't.
In regards to your friend, for the engagement party, if she isn't going to know anyone else at the party besides you, I would probably be more open to allowing her to bring one guest with the understanding that you won't be able to extend an invite to that person to the wedding.
Please keep the advice and opinions I didn't ask for though
You are on a wedding etiquette board and you mentioned the possibility of B-listing, which is a big no-no here. You will get opinions on that because we do not want you to offend your guests.
Btw, if you B-list, people will talk, I was at a wedding where people were B-listed and they knew and they told everyone. Sadly, it was only one of of quite a few etiquette missteps.
Sometimes the opinions and perspectives of others can provide valuable insights into a situation and help come up with alternatives that might work better than whatever you've been doing. Accepting the possibility of that could help you.
Ummm...neither my FI nor I know them. Maybe it's just me, but I'd rather save $100 than spend it on someone who I don't know AT ALL. It's not like it's their spouse, boyfriend, girlfriend, what have you. It's a random friend. If I have to keep my number to a certain amount, I'd much rather invite someone I know.
ETA: I see where you're coming from. I guess in my mind, I wouldn't want to go to someone's wedding I don't know. I would feel weird and bad that they are spending money to entertain someone they don't know.
If invitations are non-transferrable as has been stated many times and I invite Sally and Joe Smith, how is it okay for Sally to substitute Jane Doe for Joe Smith? Not trying to be argumentative, just asking for clarification. How is that any different than me saying, "I can't make it, so I'm sending my cousin in my place?"
I'm only asking because I feel like I'm getting mixed signals.
Absolutely. Thank you for the clarification. I think part of it also was that I was not clear in my initial (or subsequent) postings.
My first friend who is bringing a mutual friend, that's completely fine with me. Her husband is a doctor and on-call. Because we are getting married out of town, he can't come. She is using another friend as her "date" but other friend was going to be invited anyway. (And if it worked out for her husband to be able to come, it wouldn't be an issue.)
Second friend just volunteered that she is bringing someone other than her SO. That's the one that got my panties in a twist. And why I asked about the transferring of invites.
I completely agree, if I don't know my guests' spouse or partner then that's ok because I wouldn't expect them to come alone but a random friend isn't ok to me, especially since like I mentioned, the invitation is for the family.
I'm doing the same thing as far as single friends bringing dates, I did take that into consideration, but I think the situation I'm asking about is different because it's my niece inviting a random friend when the invitation is meant for the family