Generally, if you refused to give someone property that belongs to him and that you have no right over, you may be liable for conversion. Your brother-in-law may be entitled to sue you for the fair market value of the the property if you sell or dispose of it. You probably have no right to sell or dispose of it unless you give him an opportunity to get the property back and he doesn't come get the items within a reasonable amount of time. You may want to see if there is a mutual friend or family member who is willing to store the items temporarily until the brother-in-law can pick them up.
This answer is offered as a public service for general information only and should not be relied upon as legal advice.