From Muppet Wiki

Ernestine is Ernie's baby cousin, who occasionally appeared with Ernie in Sesame Street inserts. In one skit, Ernie described the differences between them, but it was revealed that they share the same laugh (EKA: Episode 2123). In another skit, Ernie tried to get Ernestine to say his name, which she failed to do, instead blurting out "Bert!" (EKA: Episode 2615)

Ernestine also appeared in the video The Best of Ernie and Bert and on Play With Me Sesame.