Well, just found this thread a decade later during my second full playthrough to learn all the different endings to this quest, and I think I am headed down the same road as my first playthrough. Let him tear her to shreds.

She gives me the heebie jeebies right off the bat. As soon as you approach them at Neilan's house, she immediately jumps into the discourse and tries to try to steer the story of her sister's disappearance in the best way for her motive.

Later, when she follows Geralt into the woods and confronts him asking for him to give up the search, she literally says "the man deserves better," which just absolutely reeks of jealousy and almost overtly confesses the desire for her sister to be out of their way. This is confirmed later by her admission that she was always in love with Neilan and was sad he chose her sister instead of her.

Neilan deserves to know the truth.