This unnamed woman is the sister of Penny Hofstadter and Randall and the other daughter of Wyatt and Susan. She has never appeared on the show while her parents and brother. She is possibly the mother of the nephew Penny mentions having and it is also mentioned that she shot her husband while they were both drunk.
When Penny was talking to her mother about her engagement to Leonard in "The Status Quo Combustion", it was implied that her sister was pregnant when she got married which is later confirmed in "The Bachelor Party Corrosion" where Penny mentions that her water broke at the wedding reception after she and her father started their father-daughter wedding dance.
In "The Parking Spot Escalation", she is mentioned to have given her sister her first bikini wax using melted crayolas and duct tape. The experience traumatized Penny to the point that she can no longer see a box of crayons without crossing her legs.