Toni, my heart goes out to you and yours! (((Hugs!)))
I agree with others that the home life is the problem, not gs. But you can't solve that - not on your own. You can only help gs by providing him with a stable, loving environment. Bravo to you for doing that!
Sorry gd is jealous about the weekdays. Perhaps you can explain you take gs to reduce the drama in her home.
Have you suggested family counseling? Dd is more likely to listen to a professional than to you, I'm afraid. And the counselor might know how to encourage her to seek help for her depression.
If she won't seek family counseling either, I agree, you may have to contact Social Services. Your gc should not have to grow up in that chaotic environment. Something has to change, and they might be able to make it happen.
If dd knows or suspects you called, however, it may hurt your relationship with her. But that may be worth it if it helps her family. Up to you, of course. Please mull it over carefully.
Meanwhile, you need to take care of yourself. Maybe make calls at work off limits, for one thing. Also, can you afford to cut back to working part time? And, perhaps, you should seek counseling for yourself, to help you cope and sort out when you should/shouldn't say "No."
Best to you!