How is it possible to be just between two wives in financial expenditure despite the difference in household expenses, and is it obligatory to spend on the children of one of the wives if they are not his?

Justice between wives is obligatory in financial support (nafaqah) and overnight stays, unless one of them waives her right. Justice among children is obligatory. As for the children of your wife from another man, their financial support is not obligatory upon you unless there is no one else to provide for them. There is no harm in the difference in financial support between one household and another, depending on the number of individuals, so the increase in financial support would be commensurate with the increase in individuals.