Sharon and Nina were once friends, but that all changed when Nina found her dolls with their heads chopped off, and blamed Sharon even though it was her cousin Petra, revealed in the episode "Clean Slate". Ever since, Nina's been doing her best to wreck Sharon's life and those who dare attach themselves to her. For example, Nina was the one who stuck Sharon with the nickname "Braceface".
Nina has helped Sharon on a few occasions though like in the episode "Lorenza" Nina helped Sharon look for the dog when, Lorenza went missing and later on Sharon gives Lorenza to Nina when she revealed why she wanted a dog.
Nina still cares about Sharon, and even saves her life when she almost dies.