Please don't do anything where if he's innocent the issue of you snooping through his stuff becomes detrimental to your relationship. Play it straight. Sit him down. He's had time to calm his nerves. Ask him to spill. Tell him how uncharacteristic this is on his behalf. If he stalls more it makes no difference what you find. He's guilty of something. If he gives a lame excuse like food poisoning or feeling overwhelmed, Then go to town on his privacy. Because by then he had all the opportunities to come clean. Same goes for your sister. These are two people very important in your life. Burning a bridge is easier when exhausting all reasonable actions. Also grab some of his DNA for future testing...Good luck. I feel your pain