Right now there are suspicions and behavior that do warrant attention.
The fear of "damaging family relationships" should not deter the exploration of this 18-year-old's behavior toward her little sister and her father. Dark family secrets are always better exposed than perpetuated.
I would recommend that a skilled family therapist who has treated sexual abuse within families begin treating this family. It is important that the 18-year-old bond with and trust this therapist, as she is the one who has sent up the red flags about possible sexual abuse by her father.
This is a most sensitive matter and the choice of therapist(s) is crucial to helping this family. The family, and especially the older daughter, must interview therapists until they find the right match for themselves.
Life should not continue for this family amidst the ongoing suspicions of paternal sexual abuse and the older daughter's frantic attempts to keep her younger sister away from her father. Intervention is needed now. Please help this family get the best professional assistance as soon as possible. Thank you for helping a family in need.