I wish that I knew the answers to this.
My own 17-yr. old has a bit of a cavalier attitude about school, though he IS going, and IS finishing, he figures it's more of a placating thing, though he knows in order to get his higher education, which I believe he's stoked about, he needs to jump through these hoops.
I think in the case of the older boy, as long as she can provide him with some options, and let him know he can always get back to his GED if he comes to his senses. This is free and he should take it while it lasts. Sister needs to figure out his game plan and get him on that path, he's perched on the edge of the nest at his own choosing, and so needs to explain to her how he's planning on supporting himself. Since I don't know what's going on in his mind (does he have job prospects, or specialized training in mind, vocational school?)
Maybe she can get some counselling for both situations, the one with the younger girl is most concerning to me at this point b/c of the drugs and her age.
At that age, she is a minor child, and she needs to get active in protecting that girl as much as she can. She needs counseling for herself as well as for the daughter. This is a horrible situation to go it as a single parent, and that dad should be flogged for the deadbeat he is. I feel terrible for her.
I'm sorry but no amount of good looks will protect you from being OD'ed, drugged, raped, left for useless, exploited, and even disfigured and then where will she be? I hope they can talk some sense to her, or intervene somehow. I think Ala-Teen may have a good resource(s) for your sister if need be ,and for sure many of them have been down the road and can help.
Liz