heres my advice ,
Ive been there done that been that kid seen that kid, altho i was not working. Had no money to blow but i had the party's and mom couldnt stop me. But at a much younger age. Poor home life i had and no role models, was on streets at age of 13 and grew up real fast i am married with 3 kids and another on way we live a humble life now and are very happy. Im sorry to say it "tough love" is the answer Unfortunatly, if they get into drugs it can just continue. All i had to do was look at my two eldest siblings and see what drugs did to them and i shaped up real quick, that was not the life i wanted.
My eldest sister even now has not changed her ways at age 32. We didnt have tough love at home nor a mother whom was capable like you. You sound like a VERY caring loving devoted mother WELL DONE!
You need to put your foot down NO FOOD if they dont pay rent. Take the fuzes out of there granny flat till they pay rent. Put a lock on the laundry door as well! Why should they be getting everything and not contributing. Heres the thing they got it tooo good. Why pay and clean when they can have it all and keep there cash and not lift a finger. If granny flat has its own meter box disconnect it they can pay the reconnection fee. Un fair. They need to learn to live in the real world. In the real world dont pay, you dont get utilities. At the age of 16 i was renting my own apartment and putting myself back through school. Its possible for them to change but they need a reality grip.

Your doing a great job! This in no way shape or form reflects on you we all have choices and those two are making theirs. Im proud of you for asking for advice and being bold enough to do so WELL DONE!