Fathwa, - My youngest daughter is not my ex-husband's

Question:
I am a revert of 2 years. Alhamdullilah! But before I reverted I made
a lot of mistakes. I fell pregnant to a person I hardly knew. After
this fact I went back to my ex-husband and he took her as his own and
thinks of her as his daughter. At the moment she is living half a year
with him and half a year with me. She is only 4 years old. I made a
promise that I would not take her from him until the year 2007. He has
since reverted and has just married again . His new wife doesn't want
our children. My questions are, is he mahram to her? And is it haram
for her to live with him and her half siblings? I would really love to
have her with me, but taking her away from her siblings is a big
worry.
Answer:
In the Name of Allah, the Gracious, the Merciful.
Praise be to Allah.
May the peace and blessings of Allah shower upon our beloved Messenger.
Dear Sister,
I pray this message finds you and your family in good health and spirits.
This situation is a delicate one, so I advise you to proceed with much
thought to everyone's wellbeing.
It sounds like your ex-husband knows that he is not the biological
father of your daughter, but has chosen to take her in anyway. His
attitude is most commendable.
Even though he is not your daughter's biological father, he is still
her mahram provided he had sexual intercourse with you. Thus, it is
impermissible for him to marry her and he must treat her as if she is
his daughter. [Reliance of the Traveller, m6.1]
It is not unlawful for your daughter to live with her stepfather and
half siblings. However, you mentioned the fact that he has remarried
to a woman who does not want your children. Please think of your
daughter's wellbeing. She may be far better off living with you, and
visiting her stepfather from time to time. I would definitely suggest
that she come to live with you by the time she enters puberty.
I also suggest you and your ex-husband take this case to a qualified
local scholar who can suggest an arrangement that is to the benefit of
all. Remember: your daughter's wellbeing is paramount. Since it sounds
like your ex-husband is the only father she has ever known, it may be
wise to encourage her to maintain ties with him, provided that Islamic
decorum is observed.
Please see the following related answer on stepfather-stepdaughter
relationships:
Covering in Front of Non-Muslim Stepfather
And Allah knows best.