@Vmpslushie
I guess it was something like that:
1) Mother sells older sister (thats why scars)
2) Mother sells younger sister too
3) Older sister finds a way out of slavery and becomes slavetrader herself
4) Older sister finds the younger one, pulls her out of streets and sells her to her friend( she knew Kujou before at least) under special contract which probably means "you get her for free , but you must take care of her" (Mistress probably is not that wealthy to buy slaves)
That kinda makes sence . The only question is "why the hell she didn`t took care of Kaede herself?".