You talk with the guy

"This world is full of problems, lad" You say sternly "You may want them or not, but you will find a lot of problems. Go in. We need to have a serious talk. She will wait outside"

"Yes sir" He follows you inside and you close the door.

"I ask you answer. Don't waste my time. I'll start with a simple question. What is your name?" You do sound as a professional interrogator. This new world changed so much in you

"I am Adam. And she is Sara" He answers quickly

"Is she your sister?"

"Yes. We are twins"

"How old you are?" You keep intimidating him with a pistol, treating him like a dangerous spy or something like that

"I turned eighteen three weeks ago"

"Do you fuck your sister?" You simply need to ask this for confirming or denying your virus theory.

"How dare you to ask this!?" The boy is clearly offended and shows some character for the first time.

"I ask, you answer!" You point the gun just between his eyes "Do you fuck your hot little sister?"

"NO!" His reaction is too aggressive. Either he is lying or...

"But you do want her since the virus hit?" You ask looking straight in his eyes "Don't lie to me. It is pointless. I am not a full"

"How do you know? " He lowers his eyes in shame" I am growing crazy with nasty fantasies. I never considered doing something sexual with sister but nowadays... Please don't tell her"

Your theory is backed by one more fact now. It is definitely interesting but you still need to decide what to do with with Adam and his sweet sister.