Its very difficult living in a house with adults, especially if some aren't relatives like this renter. The big elephant in the room as I recall is, this is AJ's house and he's middle age. It can be difficult living with an adult child. How do you see the future for yourself? Any plans to live on your own, so he can take care of himself?

Regarding the renter. Does the renter pay for meal preparation as well? Do you really want to cook for AJ who's an adult and the renters? Its a hard call to make regarding what to say to the renter. One thing for certain is - AJ shouldn't be making a fuss in front of the renter because that teaches him its okay to argue with you.