what rights does a spouse have to property shared by my husband and his brother if my husband dies

It depends on how they hold title, and a number of other factors. If your husband and his brother own the property as joint tenants, then if your husband dies his brother will own the entire property.  You will not have any interest in the property. If your husband and his brother own the property as tenants in common, then if your husband dies your interest will depend on whether the property interest is community, separate, or quasi-community property; if your husband has a will; and what other heirs your husband has.