Press ctrl+b. Click your brother nick in that menu and set his color to something. For my explanation let it be green.
Now rightclick your claim to open claim menu. Click on green color then tick boxes saying Tresspassing(to allow moving on your claim), Larceny(to let him pick up items), Vandalism(to let him destroy stuff/dig). Press buy button to save it.
Now your brother can do everything on your claim
(if he have crime debuff visible in left top corner then tell him to wait and when its gone he can enter you claim)