Is it permissible in Islamic law for the brother of the deceased husband – who is infertile – to marry both wives of his brother in one marriage, and who would be the legal guardian (wali) in the marriage contract?

There is no objection for the brother to marry the wife of his brother, if there is no legal impediment such as breastfeeding (suckling) or the like, or if he combines them before the end of the waiting period (iddah). The guardian of each woman should marry her off.