Apostrophes are used for possession. If something is plural, but doesnt possess anything, then it doesnt need an apostrophe. In this sentence, members of the Curtis family dont possess anything, so their last name doesnt need an apostrophe. Because there are multiple members of the family sending the card, however, the last name needs to be plural. The rule is that if a word ends in an s and needs to be plural (but not possessive), then we add an es at the end. So, Curtises is correct.