I am asking on behalf of my mother, my nephew lives with us practically and his mother does to, but she just ups and leaves all hours of the day and night and doesn't tell anyone she's gone with who or where and she leaves for days and weeks on end, this leaves her child we all look after him and take him to school as she isn't here to do so, we clothe and feed him and then she turns up weeks/days later without batting an eye. I feel as his auntie that enough is enough and I want to get as much information for his grandmother about getting custody of him. Could this be classed as neglegance? And could we possibly have a case?

Yes! Consult an attorney and file for custody.