I think the answer really depends on your story's take on the status of the frog-pig relationship. In KG, where Kermit and Piggy live in wedded bliss (nice town, that), it's "Aunt Piggy" to recognise her status as Kermit's lawfully wedded wife. In WOTG, it's "Miss Piggy" to Robin and most everyone else. If the frog and pig are not trothed in your story, I see no reason for Robin to call her Aunt Piggy. Just my two cents.