Is it normal for a 11 year old boy to be obsessed with a knife collection and making things like shanks?

Ok.. my little brother who will be 12 next month has started this "knife collection" like any knife he sees he has to have it, and he makes crap out of razor blades. For instance the other day when i went to visit, he had taken a magic marker and taken the bottom out and the ink tube thing and some how managed to stick a razor in the bottom of the lid and when it was closed it looked just like a normal marker but when he took the lid off the turned it around he had like a freakin shank, which he proceed to throw in the wall. He spends like part of the day on Youtube learning how to make crap and looking for knifes on ebay. Im just desperate to know if this is normal, i mean my mom had him and his twin sister really late im life and shes not at the best of health so she just seems to yell at him and tell him not to do it then he just does it anyway because he knows nothings going to happen. I just need to know if this is normal and would i be out of line to sit my mother down and confront her about doing something about his behavior, I mean hes where if his twin sister tells on him he punched her in the nose, its just not right.....