Dear Big Sis,
I think perhaps it is not just OCD here but plain and simple common courtesy that your bro is lacking in. I am like you and my computer is my link to my outside world and helps me stay in control as I know it is here and I can feel comfortable and fufill a need for myself as well as helping others.
It has come to a moment in time where you need to sit down with your bro and tell him how the situation is making you feel. Remember to use the "When you do this it makes me feel" not the accusatory "You are making me crazy" comments.
Make a list if you must but expecting him to behave in a way that does not go against your way of living while he and his family is are staying at your house is not an unrealistic request. He is accepting your hospitality and therefore should be stepping up to the plate to offer his assistance in chores or asking what works for you and your own family.
Make sure you add an expectation..........No Arguing or Loud discussions while inside your premises. It is your home, it is your special place and do not put up with the unexceptable behavior, be assertive, hold the line and do not feel any guilt.
Confrontaion of any kind is tough but none will destroy you and make you feel trampled if a wrong must be righted.
Love ya my Big Sis
Lil Sis out