Is it permissible for a person of limited income, who lives with his elderly father afflicted with Alzheimer's and is his sole provider as well as for his own family, to spend from his father's pension, which his father allowed him to spend from when needed, or must he collect it for inheritance as his siblings demanded?

If a father permits his son to take from his wealth for sustenance when needed, there is no harm in it, provided the father is of sound health and mind. However, if he is afflicted with dementia, then his guardian is the one who manages his wealth, spending on him in a reasonable manner, and preserving any surplus to become an inheritance after his death.