I am very sorry for your aunt's illness and the upsetness that your grandmother is suffering from.
Your grandmother's dreams are primarly stemming from the concerns she has and are manifesting themselves in ways that are likely deeply meaningful to her personally in a spiritual sense. Her dreams would be modeled in a way that is likely based on her belief system, at least in some degree. When these challenges come at us we also tend to develop more definition in our belief systems and may search - even by way of dreaming.
Some hold that such dreams are truly 'visits' - and no observer can prove or disprove that - it is a matter of the dreamer's own faith and beliefs as to what to make of such a thing. One thing is certain - these can be quite real - and they nearly always cause one to wonder or ask just what they are all about. One thing is for certain: the dreamer is obviously not comfortable with a challenging circumstance and is trying to cope with a trying situation.
That is certainly true in your grandmother's case. As she struggles with her feelings and the stress of this situation, she fears for your aunt's survival - and it becomes logical that those who have gone on now come seeking her. What may be comforting in the middle of it all is the familiarity she has for her deceased mother and the dog - these would be positive creatures of spirit. That said, of course she still wrestles with the very hard prospect of being faced with your aunt's illness and possible peril in this life - no one who loves another ever wants to let go, so this is a mixed 'blessing', if you will.
The uknowns that visit in this dream may represent the mystery of what lies beyond our comprehension. That can be very discomforting - but perhaps can also be taken as a call for your grandmother to seek some spiritual guidance by which she might learn and gain some comfort by maturing her own personal spiritual views.
All in all, there is no comfort I can give - your aunt sounds gravely ill and I can only hope for the best for you all. Again, I am sorry for that - and for what your grandmother is going through. Perhaps some of these observations can at least get her to a point of considering her own spiritual understandings more directly and finding some comfort there. She may at least come to lift the veil these dreams seem to drape over her and find better footing in terms of her own beliefs and eventually her feelings as well.
I hope some of this may help - thoughts and prayers are with you and yours for healing and comfort.
All the best to you.