As Dakhoth said, the entire Lass family had issues. Reggie always cared about her sister but when she was alive, George didn't pay much attention to her family at all. Once George died, Reggie was struggling to try to keep her alive in her mind and heart. Her mom, on the other hand, was trying to put it behind them.
From Reggie's point of view, it seemed like her mom was trying to bury the past and ignore it (From not letting Reggie wear any of George's old clothes or have any of her old possessions, etc). Her being a spoiled brat was her acting out against her mother who she perceived as probably not caring enough about George or that George was dead. They both handled her death differently and their manners of dealing with it were almost polar opposites, which is where the conflict between them came from.
Plus, you have to remember that George DID pop in from time to time on her family, and she broke a ton of rules in things she did: Returned her toy/doll thing (Which you later see sitting on Reggie's nightstand), taking JD/frog to them, and so forth. Reggie wanted her sister to be alive in some way moreso than simply remembering her, and she saw all these little things as evidence that George was still around, looking out for her (Of course, she was actually right about that) -- so she would naturally get upset and conflict with her mother when her mother said or did something to suggest otherwise.
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