Attend wedding without fiancé?

My parents received and invitation to my cousins wedding. It was addressed to just my parents with only their names on the rsvp card filled in, but 5 seats reserved. Now we are a family of 5, counting my younger 2 siblings and I. My confusion comes knowing that my fiancé is not invited but they are well acquainted. Just recently saw her at my shower and she was very kind. I don't want to ask either Since FH has a bachelor party that day anyway because we considered this happening because its happened already before. FH was kidding around saying we won't invite them then but of course we will, they will be married. Her shower is tomorrow, I'm not really up to going but will. Should I attend the wedding anyway? Without FH? Do I mention that he wasn't invited?