I think this will do what you want.
	- 
		Click on your brother
 
	- 
		Click on Change or Delete
 
	- 
		Click Change Parents
 
	- 
		Change the "Set the Father" field to (none) --you can just type that in.
 
	- 
		Click the Okay button
 
	- 
		Click the "Add Father" button
 
	- 
		Click "Add new father" and enter the father's details.
 
	This will show your brother as your half brother on the tree.