Praise be to Allaah.
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
Sheikh Muhammed Salih Al-Munajjid