Their father treats them and their mother harshly so they have decided to cut him off

Firstly: 

The fact that your father does not pray and loves women and
pursues them are obvious evils, the most serious of which is his not
praying, because not praying is kufr that puts a person beyond the pale of
Islam, according to the more correct of the two scholarly opinions. There is
a great deal of evidence for that, including the words of the Prophet
(peace and blessings of Allaah be upon him): “Between a man and shirk and
kufr there stands his giving up prayer.” Narrated by Muslim (82). Hence you
have to advise your father and do not despair of his repenting and being
guided. You should look for various means of advising him, such as giving
him a tape about the rulings on the one who does not pray, and seeking the
help of family and relatives who could advise him, and so on. 

The fact that he has mistreated you and is still doing so
means that you must have compassion towards him in your hearts, for if he
were to die and meet his Lord with these deeds, he would meet Him with great
sin and disobedience. 

Hence you and your siblings and family should look again at
your relationship with your father and your attitude towards him. Allaah,
may He be glorified and exalted, has commanded us to treat our parents well
and to honour them even if they are kuffaar and call us to kufr. 

Allaah says (interpretation of the meaning): 

“But if they (both) strive with you to make you join in
worship with Me others that of which you have no knowledge, then obey them
not; but behave with them in the world kindly”

[Luqmaan 31:15]

Ibraaheem (peace be upon him) debated with his mushrik father
in a polite manner, as Allaah tells us (interpretation of the meaning): 

“And mention in the Book (the Qur’aan) Ibraaheem
(Abraham). Verily, he was a man of truth, a Prophet.

42. When he said to his father: ‘O my father! Why do you
worship that which hears not, sees not and cannot avail you in anything?

43. ‘O my father! Verily, there has come to me of the
knowledge that which came not unto you. So follow me, I will guide you to
the Straight Path.

44. ‘O my father! Worship not Shaytaan (Satan). Verily,
Shaytaan (Satan) has been a rebel against the Most Gracious (Allaah).

45. “O my father! Verily, I fear lest a torment from the
Most Gracious (Allaah) should overtake you, so that you become a companion
of Shaytaan (Satan) (in the Hell‑fire).’

46. He (the father) said: ‘Do you reject my gods, O
Ibraaheem (Abraham)? If you stop not (this), I will indeed stone you. So get
away from me safely (before I punish you).’

47. Ibraaheem (Abraham) said: ‘Peace be on you! I will ask
forgiveness of my Lord for you. Verily, He is unto me Ever Most Gracious’”

[Maryam 19:41-47]

Look at the etiquette of this Prophet (peace and
blessings of Allaah be upon him), and how he addressed his mushrik father
who threatened him. This offers an eloquent lesson for those who are tested
with such fathers. 

Secondly: 

From what we have mentioned it is clear that the father’s
rights to honour and good treatment are not waived if he falls shorts and
mistreats his family. Rather everyone should fulfil the duties that he owes.
So even if he falls short, you should not fall short. Moreover, turning away
from him, forsaking him and not serving him will make him even worse, and
undoubtedly you do not want that. Many of those who sinned and then repented
have spoken of the effect of people’s forsaking them, and said how that made
things worse and made them do stupid things that they would not have done if
they had felt that they had children who loved them and respected them even
though they were harsh and behaved badly. 

No matter how bad a person is, he does not forget kind
treatment, even if outwardly he appears to be ignoring it. Hence the most
beneficial way of dealing with your father is to draw close to him no matter
how far away he seems, and to be merciful towards him no matter how harsh he
is, whilst also praying that he be guided aright. 

Rest assured that your kind treatment will not be wasted
before Allaah, and it will bear fruit for you in this world and in the
Hereafter, by Allaah’s leave. 

We ask Allaah to guide your father and set his affairs and
all your affairs straight. 

And Allaah knows best.