Relationship issues with husband and his family.

I am a brahmin girl from MP married to a gujarati brahmin. Post marriage my brother in law used to insult me every now an then. My husband never stopped him from doing so. After 1yr of marriage my son was born who is now 21months old. Soon after my marriage I realised that my mother in law is a psychiatric patient suffering from bipolar mood disorder, this condition run in my husband's family affecting his aunts as well. This was not disclosed to us before marriage. I have constantly requested my husband to get her properly treated as her behaviour sometimes scares me. But instead my husband and brother in law has always blamed me by saying that I don't want to stay with her. Even the psychiatrist whom we visited once said that now she is only verbally abusive but someday she might become physically abusive if we do not get her treated.Past 5mths I and my son staying with my mother and now my husband is threatening me of legal notice and taking my son away from me. What should I do?