Praise be to Allaah.
Spending on ones relatives may be divided into two categories:
The first category is that which is called umooday al-nasab (lit. the two pillars of lineage) i.e. the direct line of ascent and descent, not matter how far they reach. [This means parents, grandparents, great-grandparents etc, in the line of ascent, and children, grandchildren, great-grandchildren, etc., in the line of descent - Translator]. Spending on them is obligatory when the following two conditions apply:
That the one on whom money is spent is poor and does not have anything or does not have enough, and is not able to earn a living.
That the one who is spending money can afford it and has enough, more than what he and his spouse need for their own sustenance.
There is also a third condition: that both parties should share the same religion (i.e., they should both be Muslims, for example).
The second category is all other relatives apart from those in the direct line of ascent or descent. In cases where spending on them is obligatory, in addition to the two conditions mentioned above, there is a third condition: which is that the one who spends money should be a legal heir (according to shareeah) of the person on whom he is spending, i.e. he can inherit from him. On this basis, if your father and paternal uncles are able to spend, then it is their duty to spend on your grandmother and paternal aunts.
But do not forget the matter of kind treatment, for charity given to relatives is both charity and an upholding of kinship ties, and so it will bring two rewards. Do not forget what Allaah says (interpretation of the meaning):
and whatsoever you spend of anything (in Allaahs cause) He will replace it. And He is the Best of providers. [Saba 34:39]
Spending on ones relatives, especially one's mother, is one of the greatest means by virtue of which Allaah will send more provision and blessings, alongside the great reward which is with Allaah, may He be glorified and exalted.
So you should be happy that your father is spending on his mother and sisters, and you should encourage him to do that and compete with your uncles in doing this good deed.
As for the amount of money to be spent, this is determined by the ability of the person who is spending and the needs of the people on whom he is spending. Allaah says (interpretation of the meaning):
They ask you (O Muhammad) what they should spend. Say: whatever you spend of good must be for parents and kindred and orphans and al-masaakeen (the poor) and wayfarers, and whatever you do of good deeds, truly, Allaah knows it well. [al-Baqarah 2:215]
And Allaah knows best.
Sheikh Muhammed Salih Al-Munajjid