This is not a weird or uncommon set up IME.
It very much depends on what you are used to, your background etc.
Several of my extended younger family members live under this sort of arrangement.
It is what people did before SS and when SS could only offer removal into group homes.
SS will not be interested. This is just the sort of set up that they would see as a solution to any welfare concerns. Its already in place so they would have no reason to get involved.
If the OP is happy with the way things are I can see no reason why it should change. The child has a solid, dependable person in her life and also spends time with her birth mother.
Ideally she would have a stable, nurturing, solid home life but as she hasn't, surely this is the next best thing?
The massive downside is that it puts huge pressure on the OP. This must be a financial burden as well as an emotional and practical one.
You are doing a good thing OP and your input will have already made a huge impact on that little girl.
You do not have to do this though. If its becoming too much you need to sort it out with her mum.