My FI and I have planned an adults-only destination wedding. However, I have a 16yo niece in my wedding party, and her entire family (mom, much older sis, and 15yo brother) are all invited.
My FI is now upset because my 15yo nephew will be there, but his 15yo cousin can't go. And he'll be in the same position because his mom and much older brother will be invited. Ok, so I told him we'll lower the age limit to 15yo. But now, he wants his other cousin, who's 5 to go because she'll feel left out. If we invite her then all of my cousins' children will need to be invited too. And our intimate adults-only wedding turns into a giant family wedding, which we really don't want and really can't afford.
He said we need to come up with a logical reason why the age limit is 15yo and why we are alienating our younger extended family members. Apparently, "we couldn't invite everyone we wanted so we had to draw the line somewhere." isn't good enough. Any other ideas? He's really worried that strained relationships will get worse after this, and that I should just disinvite my niece (kick her out of the wedding party) and my nephew. But I can't do that!

*Edit: Some more information came out that changes this entire scenario. But, I'll leave it up in case anyone finds themselves in a similar situation.
"There is always some madness in love. But there is also always some reason in madness."
-Friedrich Nietzsche, "On Reading and Writing"