Yakface

Yackface Acachalla appears in LAVENDER TOWN! -- Gmod FAN CHALLENGES Episode 15!. She is extremely unattractive, practically deformed, which is probably why she is named/nicknamed that. She is the sister of Billy.

Billy
According to the worst camera man in the world, she is Billy's unknown sister that no one's ever seen before. It is not known whether he is her blood- or step-brother.

Gertrude and Papa Acachalla
Though not explicitly said, it is likely that she is the step-daughter of Papa and Gertrude (or maybe blood-daughter).

Sally
Though not explicitly said, Sally is likely the step-sister of Yackface.

Trivia

 * Since in some episodes say that Papa and Gertrude are siblings and some say they are married, it has been theorized that they are both siblings and married and Yackface is their deformed blood-daughter. If so, they probably hid her away because of her ugliness, which would be why no one has seen her before.
 * Yackface may be the blood-sister of Billy and did not get abandoned at Papa's 711 by their parents like Billy did and, so, was not adopted by him and is not Gertrude's and Papa's step-daughter and not Sally's step-sister.