So, we are left with two possibilities; one of the men was dating the sister and then killed everyone or one of the men was dating one of the other men and then killed everyone.
Am I so for off base in the realm of possibilities?
<><><><><
I will say yes. You are off base, not for speculating as to the reasons, but being so binary in your thinking that you believe there were only 2 possibilities.
Am I not allowed to speculate?