Does the grandmother know that this is happening?
I am not an expert, my boys are only just teens, but I did used to work as the residential support at a project for young women with behavioural problems. This doesn't make me an expert or anything, but I do have some small experience with families in crisis and with troubled teenage girls.
If the grandmother has agreed to take her, then I would suggest you do nothing beyond supporting them all and perhaps finding out where they can go for help at this point. If she's agreed to help out in order to give them both some space then that's ok. They can work from there, get some help, get talking.
If she's just planning on dumping her there without agreement, then the grandmother must choose whether to accept that and turn it into the above, or phone social services and report the child as abandoned or something.
If I had a say, which I know I do not, I would advise against telling social services she's abandoned her at her grandmothers.
Your sil should not dismiss these classes. She should move heaven and earth to make it work. Find out if she can swap shifts, or if there are classes anywhere else at other times, etc. She has to make the effort. She's the adult here and if she's not in control or doesn't understand how to parent a teen, or is really struggling - the onus is upon her to learn and to change.