Ahh rivalries.

When Sweet Bailey and Little Lady were kids, the former got a lot of compliments for being beautiful, friendly, kind and smart while Lady was just the pretty one. Her little sister got a lot more attention since it looked like she’d be going places. It was a petty little jealousy for pretty Little Lady and it still exists to this day with now Doctor Sweet Bailey being a well respected Doctor and Medical Lecturer.
Lady used to try to sabotage her sister in any way possible and one day it worked.
After the “event” AJ kicked Little Lady out of the home and sent her to live with Starlight Glimmer in Canterlot.

To this day, Lady utterly loathes Sweet Bailey
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Little Lady isn’t this way with other people. It’s a sibling rivalry that has been allowed to fester for too long.
While she may believe Sweetie is perfect, her little sis has a ton of personal conundrums. She sacrificed her social and love life to help others and is an aggressive insomniac and workaholic.
Her only friend in the world is a family friends daughter Babe Rootbeer who is more like a little sister to her

What can I say? We all have our share of issues