Given the family dynamics, the possibility of sibling incest isn't all that shocking to me. I have always contended that Casey exhibits the behaviors of someone who has experienced sexual abuse as a child.
The fact that Casey had lots of boyfriends and was 19 does not preclude the possibility of incest. In fact it makes it more feasible, IMO, since sexual promiscuity is one of the hallmark acting-out behaviors of sexual abuse victims.
Look at grandma Cindy's pathology as it expresses itself outwardly. There are deep deep issues in this family that would provide a textbook scenario for sibling incest. It is not just about sex, it is about complex family dysfunction.
Incest is an ugly topic for society to look at, but it is a reality that exists in many homes in America. Statistics indicate that as many as one woman in 8 is incestuously abused before age 14 and about 1/3 of those perpetrators are siblings. Both siblings would be considered victims in most situations. I would not rule it out in this case.
As an advocate for sexual abuse survivors, I am sorry to see that this subject sickens people instead of prompting them to learn more about this insidious form of the victimization of children.
Of course the child resulting from incest has a higher chance of health issues since they could inherit bad genes two-fold from the bloodline. That is true, but it would not necessarily manifest in a huge way. Generations of inbreeding could create some significant problems for sure, though.
$.02