I am sorry this happened and you are in the middle of this. First off what happened to your mum is not your fault. It is th fault of the Nurse and they should be called on it. I don't know if your mum has a Social Worker working with you, your sister, and your mum but this should definitely be reported to the Social Worker if your mum has one. If not you should definitely report it to the Head Nurse of the unit or the Administrator of the hospital. That was neglect what those Nurses did and they know it. It is definitely not your fault.

I also hope you can use this to get together more with your sister and continue to possible mend the relationship with your sister.