That has to suck >.<
I suggest maybe talking it over with your mom or dad and explain that you love him to death, but a night where you were supposed ot have the time of your life with your friend was intruded upon by him. Make it clear that you understand he's only a little kid, but make it known that you felt hurt after missing out on what shoulda been a great time. Be mature about it and maybe talk to your brother about it too? I dunno how old he is so I'm not sure if that would work, but it's worth a shot!
Plus, why do you have to let him mod your forum? Did your mom make you? If anythign you should explain to your mom that sometimes you HAVE to have things to yourself. Make it clear that is had Nothing against your brother, but let her know that you need some things to yourself. Try to show her that you need these types of things to relax and breathe a little. If you can say it just right she should understand and help out.