I'm probably a bit late .
The party wall act deals with party walls and other structures that are on or near the boundary ,plus a few other situations
If your worried about his wall the party wall act allows you to underpin his wall in order to safeguard it.
If the foundations for it cross the boundary and are in the way of your brothers foundations the party wall act allows you to remove the parts that are on your land, provided you do not compromise the structural integrity of the wall ,which would most likely mean underpinning it.
If your foundations are going lower than the walls foundations or those of his house and are within 3 meters of his those foundations your are meant to send a party wall notice. If you don't he can gain an injunction halting the work until you send the appropriate notices.