IIRC, Mrs. Winkfield's father is a pastor, so he could have a strong influence as both a parental and a religious authority. I'll try to track this information down tomorrow if no one else posts something in the meantime.
So far I have no information on Sandra Chatham's nursing specialization. I think that it would be very difficult to be in a situation of treating your grandhild and trying to separate your professional persona from your vibrant role as a grandparent. Perhaps a reason that doctors, nurses, et al do not work in a professional capacity with their families. JMO, but I would guess it's easier for people not related to a patient to maintain a professional detachment and objectivity during whatever procedures are required for the treatment of that patient. However, I can see why Ms. Chatham would be there as an extra pair of eyes to watch over Jahi, and as a loving pair of arms to comfort and soothe her grandchild and support her daughter.
According to the Visitor's Information Sheet for Childrens' Hospital Oakland, parents may visit any time but only one parent at a time can stay overnight with their child, and a limit of four visitors per room is in place. The sheet notes that special guidelines for the Intensive Care Nursery, the Intensive Care Unit, and the Immunocompromised Unit are posted in those areas.
http://www.childrenshospitaloakland.org/main/visitor-info.aspx
The hospital allowed 4 of the family to stay overnight and made other exceptions to the rules in order to help the family spend time with Jahi.