Would like to know how to handle someone in the family bashing you on facebook?

My sister in law died the middle of May from cancer. Her two boys and her aunt decided to have a memorial for her with the typuical family reunion that occurs yearly. So it is to take place July 2. Their are 4 sisters and one other brother, besides my husband. None of them are going to the memorial. My son talked with his cousin, who bashed me on facebook, until she got my husbands phone number from him and then no more communication with her. She called my husband when his sister died and then texted him the date of the memorial. He never responded. She tells everyone on facebook that if I, his wife, had anything to do with it he would not be going. To me, I can't understand how they don't understand how he feels. He only called his sister after he was prodded to by them. His family doesn't like me and has done things in the past to alienate him from them. Now he has a new cell phone, since April, and no one in the family has the number except her because my son gave it to her. She told my son they would like to him and my husband to come see her before she dies, never mentioning me or my other two sons. They have a history of using my kids to get to my husband. It now is all over the internet that I am blamed for my husband not attending. I do not have a facebook account. My son just put it on my phone, when he didn't have a phone and so there is a part of my phone where I can see it. How do I handle this? All the family has one of these accounts and if they never felt bad about me before they will now. I told my husband he has to straighten this out, but how?