May I ask what is your relationship to the mother and child .? Many families do sleep together all round the world ,it is a choice I doubt if they are all abused,it may be preferable for a child to be independant by this age ....#1 it is not unhealthy but maybe best for the child to have her own bed. #2 No it is not ..it seems a consensus that small children who do it a lot may have had incidence of abuse .
I have a platonic 30 years friendship with the mother. I don't think there is cultural issue in here. This new arragment started just two weeks ago.
then other than advise the mom I dont see what you can do about the way you feel, what culture are you referring to ? Perhaps there is a reason the Mom has started this recently, has the daughter been upset by something ?