It would be best that you tell your sister so she has time to come to grips with what your dad has. Since she lives with him she will have to help him with the ALS for as much as she can until it becomes a case where home care is needed. Knowladge is power.
Sorry to hear of your father and his ALS. I had neighbour who had this and it was tough watching the illness take control over him. Be strong and positive for your dad and your sister as they will need all the support they can get.
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