Hi Sarah,
Hard to advise when I don't know about your current relationship with your father but I would think that if you are usually close it may be nice for him to hear from you that you care about your aunty and you care about him.
He is probably acting very matter of fact because he has dealt with alzheimers before (his mum) and probably has spent many a year already concerned for his sister due to her down's syndrome. I would think maybe he's always had to be strong?
So don't be put off if he doesn't necessarily want to talk back as it may be hard for him to 'be strong' if he actually does have to talk about it, but I can't see what harm it would do to just tell him that you worry about your aunt and worry about him, perhaps you could tell him you don't want him to worry about you too but that you want him to know you care about him and hope its not all too hard for him as you imagine it would have to be. I know this doesn't help you immediately if you are looking for someone to share your grief with, but if he knows you are concerned you may just find that opening up to him will allow you both to talk about it?
My Dad has dementia and that has made me realise that no real harm can come from telling a family member that you love them or worry about them or worry about other family members. I also have a mother who can be rather matter of fact, and sometimes she brushes off my emotional moments as if I am being silly and over emotional but I don't take it to heart and am just glad that whether my thoughts and feelings are outwardly scorned, tut tutted or smiled at or not, they have been said, so my Mum and my Dad do know I care deep down. I'd rather that than them thinking I was cold and uncaring!
P.S. Often actually talking to someone about this stuff can be rather embarassing and awkward and the words don't come out too well and first up you may think 'oh my gawd why did I even try to say something' or worse yet you may find your words didn't come out right at all...but this is your Dad and he will eventually get it. When my Dad was first getting ill I didn't know how to talk about it with him, but he knew I knew and even though I was in my mid twenties at the time, I decided the best way I could express my concern and my worry was just to grab his hand and hold it sometimes like I would have when I was a little girl. We never discussed anything but I am glad I held his hand now and I think he was glad I held it too...I think he knew what I wanted to say.
