Alternative education for a 16 yr old daughter returning home from therapeutic boarding school with 18 year old sister still at home Yikes.

My 16 year old daughter has been away at a therapeutic boarding school for the last 14 months. She's made huge strides and will be coming home within the next few weeks. My two concerns are:
Her older sister who is 18 and still living at home (she deferred from college for a year and is taking an EMT class and working). She's extremely self centered and focused and only cares about herself and her new boyfriend of 5 months. She herself should've gone to a therapeutic boarding school when she was 16 (anxious, insecure, started dabbling in drugs, no real friends, etc) but her father and I couldn't pull the trigger....biggest regret of my parenting career. Anyhow, I'm very worried about the 16 year old coming home and seeing some of her sister's behavior etc.
The other concern I have is that I don't really have a game plan for the 16 yr old coming home. I'm certainly working on it; but going to the public high school is not a good option for her. She recognizes the unhealthy friendships she had while there. I tried getting her into a parochial school; but because of her grades (just a 2.3/4.0 GPA) the school wasn't interested. Her father and I are going to talk to the school district about possible online learning, but I'm at a loss. I'll need to keep her very busy which makes me think I'm going to have to quit my job.
Any thoughts on any of this anyone?!