Manowarr, that's not true.
I killed the sisters to finally be able to afford my spells. After doing so for a while I had no problems in Kelethin or Felwithe. I'm speaking of the Nybright sisters including Kayla Nybright,
http://wiki.project1999.com/Kayla_Nybright
The wiki reports: Factions - Dervish Cutthroats (-30); Opposing Factions - None.