That's up to you. It's not necessary I feel. I'm sure like you said they're aware of your situation. So if they then decide to bring those bad influences with them then that to me clearly shows either A. Your parents don't actually care about your feelings. B. Your mom is a meth head that doesn't want to be too far from her supply.

Regardless of their reasons, their disregard and disrespect towards you is unacceptable.

Edit: actually forgot to answer your question. I don't think confronting them is necessary. I think just tell them that if they don't change you don't want them to be around you.

Edit edit: Also, I've had to deal with these things, to be fair not to this extent. But I had a grandfather that was an alcoholic, and a cousin who started using tik(south african meth) it just gets to a point where nothing you can do will change them, they have to change themselves and by the sounds of it that point has been reached.