Dorothy and Mary both started as aides in the Galactic Senate. There they met Senator Mon Mothma. Every day the three of them would meet for lunch where they would drink margaritas, criticize the clothing choices of other Senate employees, and discuss their active love lives. As they aged, this ritual went from hot to disquieting to flat-out disgusting.
When the Rebel Alliance was formed, Mothma gave Dorothy and Mary some bullshit positions in the command structure so they'd stay close. Dorothy and Mary were underqualified and not really interested, so they spent most of their time making fun of Mothma behind her back. Dorothy also had a 23 year long affair with Mary's husband before dumping him for Mary's son. She was able to keep this from Mary until Shemp's death in the Battle of Endor.