Can he give zakaat al-fitr to his half-siblings?

Zakaat al-fitr is to be
given to the poor and needy, because of the hadeeth of Ibn ‘Abbaas (may
Allaah be pleased with him): The Messenger of Allaah (peace and
blessings of Allaah be upon him) enjoined zakaat al-fitr as a purification
for the fasting person from idle and obscene speech, and to feed the poor.
Narrated by Abu Dawood (1609) and Ibn Majaah (1827); classed as hasan by
al-Albaani in Saheeh al-Targheeb (1085). 

Ibn al-Qayyim (may Allaah
have mercy on him) said: 

It was his way to single
out the poor to receive this charity. End quote. 

Zaad al-Ma’aad
(2/21). 

Having established that,
then we know that a poor relative is more entitled to zakaah than anyone
else. 

But it is not permissible
for anyone to give his zakaah to those on whom he is obliged to spend. 

You have to spend on them
if you would inherit from them. If you would inherit from them then it is
not permissible to give them zakaah, but if you would not inherit from them
then there is nothing wrong with giving them zakaah. 

If you would inherit from
some of them and not others, then give zakaah to those from whom you would
inherit. 

You are not obliged to
spend on them unless you have wealth that is surplus to your needs and those
of your family. If that is not the case then you are not obliged to spend on
them, because you are not able to do so. In that case you may give them your
zakaah, even if you would inherit from them. 

See also the answer to question no. 106540. 

And Allaah knows best.