Oh, wow.
I understand him interfering in your relationship with somebody else, but his own brother?! That is twisted.
Our friend Mike is obsessed with you, and needs all the help he can get.
Please, keep us posted on what happens with this guy... and be sure to contact HR if he gets any creepier because you don't deserve to work in such environment :(
Is he still dating that girl? Does she know her boyfriend is a weirdo?