I would treat all cousins that are out of high school (adults) the same and extend a plus one to each, regardless if they have a current boyfriend or girlfriend. If you can't extend a plus one to all cousins for financial reasons, include a plus 1 to cousins to only those who are engaged to married or they live together, the rest would get invites without a plus one even if they have a steady girl/boy friend. Again better to include plus one to all if you apace afford it. Speaking from experience, If you treat your cousin differently it creates hurtful feelings and damages the relationship. And also remember that weddings can be hard for single adult guest especially if a long term relationship just ended or if you are not dating anyone at the time the invites are sent. If your adult cousin does not have someone special in their life they probably won't be a date in the end, but putting plus one on the invites allows them to have the choice and not feel more alone.