Have you got a local youth service or mental health service you could access? A brook clinic might be a good starting point. You could also ask social services for advice - this sounds like a private fostering arrangement do I assume you have a social worker? This should fast track you into support services.
If you have been looking after her for 28 days, you legally must notify social services and be registered as a private foster carer - it sounds like this will give you an access to support. You could contact Camhs and discuss things with them too.
Also it will be difficult I imagine for Camhs to give you information as you are not a parent, but you can write to them, explaining that she is struggling and at least suggest she is have her own therapy . Camhs is primarily for the child, so she should be their focus, even if mum is involved and if you are her significant carer, its best you are on camhs radar so you know who to call if things get tougher. It's seem best to sort things legally so if she continues to be unwell, you and she, are clear about your role. Teens suffering depression do need home support alongside therapy, and you sound like you really care so I agree its best to involve a team of professionals to protect and help both of you . best of luck.