That she wasn't close to her family doesn't mean it was only her fault. Sunset and Sunburst being siblings were only my headcanon which is not real, but I personally don't believe everything Sunset did in the past was solely her fault. First of all, she was only a filly when she was mean. There had to be a "trigger" when a little kid behaves that way. If she was "never" close with her family, even as a kid, then even her family is to blame they didn't give her the proper care, love and attention she needed (she wasn't born evil). Maybe the possibility she was neglected and extremely talented could cause her lust for power and admiration. The way she reformed (and what she said after being defeated) would explain it. But in any case, the blame can't be just one-sided when it comes to a child.