If I understand you correctly Bea, your niece is either looking after your very ill sister or the only family member actually in contact with her.
This same niece has dropped all contact with you, and has with or without her mother¨s permission control of her phone, so you are left unable to contact your sister, or get any news of her.
I am extremely uncertain whether there is anything you can do if this is the case, except report to social services that you are concerned that your sister is possibly not being looked after properly, as you are being denied access to her by her daughter.
Legally and ethically we have a duty to report to the relevant authorities if we suspct that anyone is being denied necessary care, or being abused.
Whether your niece¨s behaviour can be seen as neglect or abuse, I cannot judge, but you are obviously very worried so contacting social services is, I think, your only option.
