Essentially what the title says - I know there's a sidequest for Nassana in ME1 where you have to go to some planet and kill her sister or something another, and then in ME2 she shows up during a character's mission - but if you don't do the whole "kill her sister" thing in ME1, does that change anything at all in ME2?
There are minor changes in ME2 if you do kill her sister.
During Thane's recruitment mission, Shepard will remember how Nassana manipulated them into killing her sister while speaking with Seryna. After the squad fights their way through Nassana's guards and confronts her in the penthouse, she will say that Shepard is now finishing the job that was started with her sister, and now Shepard is here for her.
Nothing appears to change if you save her.