Frontotemporal dementia is one of the lesser known types of dementia, characterized by brain damage in the frontal and temporal lobes. The cause of this dementia, like Alzheimer's disease, has yet to be fully determined, though in some cases, there is evidence of genetic mutations. However, also like in Alzheimer's disease, some cases present with no family history.
I'm so sorry Adelejr! You are suffering along with your Nan. Most of the time, this is not hereditary, though there is much that isn't known. Some forms of the disease have a genetic link, but many, many people don't show any genetic link but get it anyway. Nan may have "mixed dementias", meaning that she could have frontal lobe as well as vascular (you mentioned mini-strokes) dementia.
Please take care of yourself, support Nan and love her as she is. Reach out to others as your are doing now. We do understand your pain, as most of us have been through a similar situation. Also, you may want to go to www.wellspouse.org to get more support for spouses.