Esmeralda is Big Bird's sister. She's never seen, but has been mentioned several times by her brother, usually in reference to her children, to whom Big Bird is an uncle.
In a repeated anecdote, Big Bird receives a letter from Esmeralda stating that she laid fourteen eggs last month and they all just hatched. Big Bird is amazed that his sister now has fourteen baby birds around the nest, and says "gee!" Because Esmeralda made sure to ask her brother to say hello to Mr. Hooper for her, Big Bird repeats "G" to himself on the way to Hooper's Store so that he'll remember. (This scenario appears almost identically in episodes 0027, 0539, and 1260, except the person asking Big Bird why he's saying "G" over and over changes from Gordon to Bob to Olivia, respectively).
Esmeralda laid another egg in Episode 0093, but she incubates it not far enough away to send a letter from, but right in Big Bird's nest. In fact, Big Bird himself babysits for his nibling while his sister goes downtown, and sits on the egg to help keep it warm. Susan is initially confused about Big Bird's literal "babysitting" until she realizes he's sitting on an egg. Upon inspection of the soon-to-be-hatchling, she notes that it's a beautiful shape. (This story appears again in Episode 0345).
In Episode 0581, Big Bird brags that his sister laid an egg yesterday and that her eggs are so big, it only takes three to make a dozen.
- The spelling of her name is inconsistent in scripts, but her first mention in Episode 0027 uses "Esmeralda".
- Episode 1571 — which takes place in between two episodes where Big Bird talks about his sister — features a story in which he and Snuffy lament over the fact that neither of them have sisters (Alice Snuffleupagus would be introduced seven years later), so they ask Timi's sister Holly to be their sister.