How to handle someone using your kids to get to your husband?

My in laws use my sons to get to my husband. They will contact them and then once they hear from my husband they do not contact my sons again. My sons do not like to have friction so they go along with them. Over the years they have caused problems to the point that my husband got a new cell phone and never called any of them to give them his new number. Now the current situation is that my husbands oldest sister died of cancer and the memorial soon and he has not responded to their text. So what happens is they get on facebook and tell the whole world that I am the reason he is not going to the memorial. Their are 4 other sisters and 1 other brother that are all not attending either. How do I handle my husband not setting things straight and them slandering me on the internet?