I'm sorry to hear about your mom. I'll second the comment that depression can have biochemical as well as situational causes. I do find it interesting that when you talk about all being well in your family, you refer to your father, your sister, and yourself, but not what your mom is doing. Was her primary focus for the past decades taking care of children and being a homemaker? If so, it is understandable that she might feel a bit purposeless now that you are grown.
One slightly geeky physics major attempts to survive life in the real world.