I can only assume that she MUST obtain evidence now that he killed someone close to her… This is your inciting event, and it needs to be front and center in the logline – start with that.
Secondly, what’s the purpose of evidence if not to use in the court of law? Her goal shouldn’t be evidence but the legal implications thereof. Also, the stakes are implied but not stated – “…sadistic steps…” is too vague. Will he kill her? Torture her? Beat her up? Kill her would be best, as it’s the highest stakes.
My try:
After her boss kills her sister, a gullible whistle-blower must obtain evidence to incriminate him and save her own life.

Lastly, a whistle-blower, by definition, is not gullible. Also, gullible, much like its cousin ‘naive’, is not the strongest description as they’re not really flaws. Gullible and naive are generally perceived as positive character traits, not flaws. And naive and gullible characters have been done too many times already, especially female ones.
DON’T GO THERE – LEAVE THAT CAN OF WORMS SHUT THIS IS ABOUT STORY, NO SOCIAL POLITICS.
What other flaws would be more original, and by extension, marketable? Could she be vengeful, selfish, or perhaps a criminal herself?

Last thing is, if you’ve mentioned her boss, why not mention her job? Describe the character using her job and a genuine flaw. This way, her characteristics can be tied into the story’s themes, and the logline will improve.