There are several thing about the opening POV sequence that don't really make sense:
Why was Michael across the street from his own house?
Why did Michael have to sneak around his own house? Yes, he had a knife with him, but no one seems to question hiding a weapon behind one's back in other movies, and no one at this point suspects Michael of being a killer.
Judith might've been surprised and embarrassed about her younger brother barging into her room while she was topless, but she was still a teenager and he was still a little kid - she should've been able to recognize that he had a knife in his hands quickly enough and been able to push him away, disarm him, etc.