I think at your mom’s age she is not going to change now and she is not going to throw your brother out. She also doesn’t need a stroke from this stress. I’m sure she is not looking for any kind of confrontation. She is sadly the person in the middle who at her age shouldn’t have to be dealing with this but her choices have gotten her here. You and your sisters are putting it on her to address when maybe it should be the sisters speaking with the brother as a united front in the best interest of your mother and her wellbeing
All of you should tell him that you are aware of his illegal activity on her property and next time any of you find pot growing you will destroy it and if it continues then his actions leave you all no choice but to get the authorities involved…..for mom’s best interest.
He will know that you all know what he's doing and he'll know the consequences if he continues.