Okay who claim or said that Lapis(?) killed her sister...? Unless they have been reading the raw chapters ahead of times that seems like a bit of a rash statement...? It sounds like to me she just is feeling survivor's guilt from how her sister had died in her place (Stolen her life by accident) to protect her sister like the other comment are suggesting. We all seen the flashbacks so unless there is a future raw chapter showing Lapis(?) stabbing her sister with a knife or something then it just guilt from surviving (Lapis(?) being saved by her sister from the assassination attempt by the Anti-nobles).
MC...Yeah, you had almost got your waifu....but sadly you you chosen the wrong dialog options and words...Bold of you to assume that Lapis(?) hasn't already kill a few dozen Anti-noble fraction member terrorist scum...Though I guess if the theory (Which is now at 50%+ possibility of happening sadly in my eyes...) is true then yeah...If you exclude any possible anti-noble terrorists she killed behind the scenes then perhaps maybe (Technically) she did accidentally 'stolen' someone's life before. MC was so close to convincing her but sadly fumble the last part without knowing....
Lapis(?) desire for revenge Versus her desire to live a happy life with MC....Which will win in the end?
Honestly though...Fk the anti-noble fraction...I would support going after corrupt evil nobles but from what we seen there are many kind righteous nobles like the prince and Lapis(?)'s family...Assuming that Lapis' family isn't actually secretly corrupt and evil. Actually went and try to assassin a little girl in the past...They are even allying with lesser nobles for more power, sounds like a bunch of parasites eager for power.