How do i stop others using my gamer card on an xbox 360?

my brother keeps using my gamer card on the xbox 360 to play games because for some reason it automaticaly signs my profile in first. and he said he just plays on any profile thats signed in. and it always happens to be mine. i managed to put a pin on my xbox live suscription but not the main account itself! help!How do i stop others using my gamer card on an xbox 360?well you go to your gamertag settings and turn off auto sign in and then you can put a password on the tag off of family settings