I would listen to that sisters advise. 
You are assuming that it will change her life.. Even if does; it is irrelevant in your decision regarding your cousin. And If your sister is not worried about her husbands reaction, then why are you?
Btw in these kind of marriages, even small issues affecting one couple tend to blow up and affect the other couple. Its natural to want to take sides with a sibling, so have you thought how that will affect your sisters or your marriage in the long term; should you go ahead?
Ps. Princess: thank you for the kind words