Originally Posted by Rowan

Sorry, but I get a strong impression that the brother is being blamed for what really is poor bike maintenance. Likely that the adjusting bolt wasn't tightened up for a while, and the OP riding it probably wore down the teeth to the point that it manifested with a really loose seat with his brother riding it.

The real lesson here is that it pays to check a bike over from time to time with the requisite allen and other wrenches, and checking for important stuff like brake cables and brake blocks.