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
It is possible that she is the step-daughter of Papa and Gertrude (or maybe blood-daughter).

Sally
It is possible that Sally is the step- or half-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, probably with Jeremy Acachalla, which would be why no one has seen her before. As well, if this is the case, Sally would be her half-sister, since Papa is Sally's father and Sally Betty Jessica is her mother (at least, this is accepted as truth at this point).
 * 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. If true, she technically would be Yackface Smigglebug.