Don't delete the toons off your brother's account. Just close it and stop using it. Then go get yourself 2 new accounts and link them to your main acct. Err. Actually daisy chain them is better.

You never know maybe your bro will see what you are up too multiboxing and he may want to return. If you delete his characters he won't have anything. Just stop using it now so you don' t have to deal with the headache later.