Hyperemesis Gravidarum is completely different to morning sickness. Many women get hospitalised for the whole term of pregnancy when they have it as they need intravenous fluids and sometimes PEG feeds.
Kate is lucky she has doctors on standby.
Given who she is and who she is carrying and having notice as she had it badly in her first pregnancy I imagine they have a good set up for it so she has everything she needs and to avoid a hospital stay unless required. An IV can be set up at home for her by private physicians if she needs it I imagine.
However much she could have people at her beck and call and private physicians on call and first class medical care, as she is close to her mum, maybe that is what she needs.