There is a link to novelupdates on the main page but only 19 chapters translated so far.
You'd have to look for the raws on google and MTL for the rest.
I think she calls her sister "useless" in that she's politically useless - the king wants his son to marry the main character to get the political capital to make his claim on the throne more stable; marrying the adopted older sister lacks that political capital, so she is politically "useless".
@Lililac I don't know, this is all my guess, but I imagine it's because Clara is adopted - she doesn't have the same blood, so marrying her doesn't give the prince the same political power that marrying a full blooded daughter does (since nobles are usually petty like that). That's my belief, at least - beyond that there isn't enough information to know. I just speculate that that's what she means because the line calling her useless happens right after she talks about how the king wants the prince to marry her for power.