Usually i am in another state than my brother, but since it is summer i am back home and he wishes to join the combine! So my question is this, if our online times overlap or anything like that will it be understood we are legal? what are some things we need to avoid, so that we arent mistaken for illegal multis?
Just register. I assume you could unregister after summer but likely you will come home for a holiday and will experience the same trouble again.
As for avoiding trouble being a multi, NEVER GIVE HIM YOUR LOGIN INFO. NO MATTER WHAT. EVER. Even if you are going away for a week and your ship gets out of hyper a day into your journey and you will lose 6 days of travel time if you don't get him to just initiate hyper while you are gone. 6 days of travel time pales to a 30 day ban.