Long story short: to keep the peace with my family, I'm inviting aunts and uncles as family units so that they can choose if my cousins come along (invitation is addressed to "The Smiths" instead of "Joe and Jane Smith"). I really don't care to have all the cousins there, except for on my mom's side as I am close to these cousins. I feel bad inviting cousins on one side and not on the other, so I'm just opening it up to both sides.
Here's where it gets hairy -- most of my cousins live at home (are under 18), so I can umbrella them in with my aunts and uncles. I am not directly inviting them -- I'm just not excluding them on the invitation. For the two cousins I have who do NOT live with their parents, do I need to extend an invitation? Hope this makes sense.