I am so very sorry for the loss of your mother.
Everyone handles loss differently. Your family should lay off and back off. You have your own grieving process to go through. Just because you are not visibly upset doesn't mean that you loved your mother any less than they did. Also, you did not kill her. I don't understand how they can even come to this conclusion? It sounds like they are taking her loss very hard and saying things out of anger/sadness. I would do my best to ignore all of that. They might apologize later. I know its hurtful, but I have a feeling that they are not themselves during this time.
Please take care of yourself. If you have to, remove yourself from the drama. Give yourself a break from people who are causing you stress. You need to take care of you.