Question Summary:
if the nikah of my father and step mother breaks, then is she still my mother?

Question Detail:

if the nikah of my father and step mother breaks, then is she still my mother?
how should i treat her?
shouyld i visit her and children from her?

Answer :

In the name of Allah, Most Gracious, Most Merciful
Assalaamu `alaykum waRahmatullahi Wabarakatoh
Your step-mother is considered your mahram now.  It is permissible for you to talk to her without any Shar’ee purdah in between.  Even if the nikah between your father and step-mother is broken, you should still treat her with kindness and good character.  This in fact is the case with any Muslims, more so if that person has become your mahram. 
As regards to her children, if the children are born from your father, then they will be your siblings and you should treat them as you would treat any other siblings.  If they are not from your father, but from a pervious husband, then they are not considered to be your siblings and a Shar’ee purdah is necessary from the daughters.  However, if she has breast-fed you, then her children will be your mahram and Shar’ee purdah is not necessary.  What ever the situation may be, a Muslim should always deal with others with a good character.  Imam Bukhari in his Sahih narrates the following hadith:
 
عن مسروق قال كنا جلوسا مع عبد الله بن عمرو يحدثنا إذ قال لم يكن رسول الله صلى الله عليه وسلم فاحشا ولا متفحشا وإنه كان يقول إن خياركم أحاسنكم أخلاقا (صحيح البخارى)
Masrooq narrates that we were sitting in the company of Abdullah bin Amar.  He narrated to us that Rasulullah (Sallalahu Alaihi Wasallam) was neither fahish (one who speaks bad words), nor Mutafahish (one who speaks obscene evil words to make people laugh).  And he used to say, “The best amongst you is he who has the best character. (Sahih al-Bukhari)
( فصل ) : وأما الفرقة الرابعة فمنكوحة الأب وأجداده من قبل أبيه وإن علوا ، . أما منكوحة الأب : فتحرم بالنص وهو قوله : { ولا تنكحوا ما نكح آباؤكم من النساء } والنكاح يذكر ويراد به العقد وسواء كان الأب دخل بها أو لا ؛ لأن اسم النكاح يقع على العقد والوطء فتحرم بكل واحد منهما على ما نذكر ؛ ولأن نكاح منكوحة الأب يفضي إلى قطيعة الرحم ؛ لأنه إذا فارقها أبوه لعله يندم فيريد أن يعيدها فإذا نكحها الابن أوحشه ذلك وأورث الضغينة ، وذلك سبب التباعد بينهما وهو تفسير قطيعة الرحم وقطع الرحم حرام فكان النكاح سر سبب الحرام وأنه تناقض فيحرم دفعا للتناقض الذي هو أثر السفه والجهل جل الله تعالى عنهما (بدائع الصنائع)
And Allah knows best
Wassalam
Ml. Ehzaz Ajmeri,
Student Darul Iftaa
Checked and Approved by:
Mufti Ebrahim Desai
Darul Iftaa, Madrassah In'aamiyyah