Difference in spending on
      two wives, and spending on children of one wife from a previous marriage
Question

                I am married to two wives.  I am not quite sure how to be fair towards them regarding financial issues.  They both live in different houses.  One has 2 children from a different marriage, and the other has one son from our marriage.  Do I have a financial obligation towards those children who are not mine?  The two households spend different amounts of money on basic necessities (food, electricity, gas, transportation....), how can I be fair towards both and that is the case?
Praise be to Allah.Treating
      wives equally with regard to spending money and time is waajib
      (obligatory), unless one of them agrees to forego some of her rights in
      favour of the other. It is also waajib to treat the children equally, but
      you do not have to spend on the children from a different marriage unless
      there is no one to spend on them, in which case their expenses are an
      obligation on all the Muslims, of whom you are one.
      
      
 If
      their accommodation differs in terms of basic necessities because one of
      the families is larger than the other, there is nothing wrong with that,
      but the additional spending involved should be related to the larger
      number of individuals concerned.
      
      
 Written
      by al-Khudayr