It sounds from what you wrote, that there may have been a miscommunication between him and his wife. Give him the benefit of the doubt, call him and have a talk. If he doesn't know what the sil said, he needs a chance to defend himself before you and your mom have a fight with him.
yes!!!!!!!!!!u can yell on him after all u r his sister. u have rits cos u love him. So just call him n have feight with him . If i was u then i ll do this.