His half-sister from his father has a half-sister from her mother. Is it permissible for him to marry her?
Question

                I have a half-sister from my father, who has a half-sister from her mother. Is it permissible for me to marry the half-sister of my sister from her mother?.
Praise be to Allah.
    Yes, that is permissible. 

    Shaykh Saalih al-Fawzaan said: 
If you have a half-sister 
    from your father, then it is permissible for you to marry her half-sister 
    from her mother, because there is no relationship between you and her 
    half-sister from her mother that would prevent that. Allaah says 
    (interpretation of the meaning):  
“All others are lawful”

    [al-Nisa’ 4:24]

    al-Muntaqa, 5/258. 
Shaykh al-Islam Ibn 
    Taymiyah (may Allaah have mercy on him) narrated that the Muslims were 
    unanimously agreed that this kind of marriage is permissible. He said: It is 
    permissible for a man’s half-sister from his mother to marry his 
    half-brother from his father. This is unanimously agreed upon among the 
    Muslims and there is no dispute concerning that. And Allaah knows best. 

    From al-Fataawa al-Kubra, 3/163.