Is she your child? if so then ask the granma and sister to make sure she sleeps in her own bed as they are enabling it, I do not think it is harmful and usually the child wants their own privacy when puberty hits .Many families sleep together quite healthily , it is up to her Mom to be comfortable with the situation and if she is not she should speak to the granma .How often does she visit them ?
What worries me about this, is that she will then never learn to sleep by herself.
I might be taking this a bit far, but i would worry about her teenage, and adult years. Will she getting herself into relationships just for the comfort of not having to sleep alone. Will she get involved with men for that sake (and all of this happens subconciously).
I would try and encourage her to start doing it - mayby start with her sleeping on the floor in the same room first, so that she is atleast in her own bed, but not alone in the room, then ask her to move into her own room maybe 1 night a week and then move it to 2 nights and so forth. she needs to learn to become independant, for the long term sake