Fake vouch?

hey guys so recently i got trade with cautioned and banned becuase of "fake vouching".
it wasn't a fake vouch because 007g is my brother. He does not live with me although but at the time he was round mine.
He was planning to buy a mazda rx8 but didnt have enough money. He quit runescape but still had gp so i thought why not buy his gp and help him out with some money?
He vouched me for it his way of saying thanks, and please tell me where in sythe rules it says this isnt allowed, to trade with ur brother.

Yours sincerely,
RSGOLDEXCHANGE

 

http://www.sythe.org/vouches-forum/412717-you-not-allowed-vouch-yourself-irl-acquaintance.html