I usually don't get involved in these threads-though I do find them entertaining.
But here two things come to mind-either
1-somebody stole this guys phone number or I.D or whatever and you may be about to unleash a bunch of crap on an unwitting victim.
2-Or this guy is really a creep-in which case, if your sister is really getting disturbing texts, scaring him away from her may only lead to his eventually hurting someone else. Meaning you should go ahead and gather your information, but let the proper authorities act upon it. you might scare him away from your family-but he will just find a new victim down the road if left unchecked. Plus, there is a possible legal can of worms you may be letting yourself in for by attempting a character assassination on him by spreading info to all his friends and family.
food for thought.