Am thinking i might ask DP's cousin to watch the girls while we go to a movie then sneakily ask her to go and get them pierced.. then DP can't say ANYTHING to me..
Getting ears pierced might be a little bit painful and I think they would want their mummy there ... nothing even comes close to cuddles from mummy (and daddy) for a little kid. It might be a bit traumatising for them to be taken by your DP's cousin for something like that and it could have a really bad impact on their relationship. It might also badly affect your DP's relationship with his cousin and cause all sorts of problems.

I can understand your reasoning for wanting your girls' ears pierced, but make it special and do it with the support your your DP