Does any children that come from the widow's previous marriage considered the same as a man's own children. Are a man's obligations towards his own children the same as what he has to the widow he may marry children?
In general: Previous children of a spouse (i.e. step children) are not considered the same as a person's own children. For example many obligations do not apply such as maintenance, accommodation, nursing, inheritance etc. Rather these are due on the biological relatives of the children.
There are however some laws which are the same for step children as for biological children, for example the prohibition of marriage extends to step children. i.e. one can not marry them.