Praise be to Allaah.
In order for fasting to be
obligatory, it is stipulated that a person be of sound mind, because the
Prophet (peace and blessings of Allaah be upon him) said: “The Pen has
been lifted from three: from the insane person until he comes back to his
senses, from the sleeper until he wakes up and from the minor until he
reaches puberty.” Narrated by Abu Dawood (4399); classed as saheeh by
al-Albaani in Saheeh Abi Dawood.
As your sister has reached
this stage or being unaware and unable to distinguish anything, she is not
obliged to fast, and she does not have to make up missed fasts or feed the
Prayer is like fasting; she
is not obliged to pray either.
Shaykh Ibn ‘Uthaymeen (may
Allaah have mercy on him) said in al-Sharh al-Mumti’ (6/202):
Anyone who is not of sound
mind is not accountable, and he does not have to do any of the duties
enjoined by Islam, such as praying, fasting, feeding the poor etc, i.e., he
does not have to do anything at all. Based on this the one who is
feeble-minded does not have to fast or feed the poor, because he has lost
the thing that qualified him to do that, namely his reason. End quote.