An final look at this film, I consider the possibilities; the reason(s) why the mother (Emily) deceived her daughter (Kate) and extorted her father, threatening charges of rape while knowing that the wound(s) were due an accident.
To say it again, why...?
Clearly the mother wanted the land/island property, so much as to implicate her father as a rapist or child molester and more, to lie to her daughter. Remember that the husband knew of her ways, the deception, but went along with it--and anything similar--for some 25 years; thus, he too was responsible (partners in crime). This matter is deep and dark, intimate and wicked, a conspiracy instigated by her and supported passively or actively by him.
Could it be that she, the mother, is mentally ill; that her behavior is symptomatic of an emotional or mental disorder? Without remorse, and insistent that her behavior was for her daughter (and not for her alone), the truth or facts of the matter are inconsequential, without account.
The daughter however is released from the shame and the longstanding lies; she is free to begin a new life (as she does), unleashed from her mother's control and conspiracy--a choice that makes for a wonderful end (or beginning) for Kate and Preston.