This topic has been a very stressful one for us. Ideally, we'd like to have a no-kids wedding, but it's causing friction. My fiancé would like to invite her 15-year-old first cousin, and I would like to invite my 1 year-old direct niece.
Finacé's Opinion: Cousin should come since they're so close, but niece should not be invited and (we'd pay for a babysitter) to avoid the baby crying during the ceremony and my sister and brother-in-law (who are in the wedding party) being distracted all day.
My Opinion: We should invite both the cousin and niece or neither of them. I'm not sure how I could explain to my sister that we made an exception to the rule, but that it was for my SO's first cousin, and not her kid.