The way your husband’s mother has behaved is strange. How can her son control her and make her act in this manner? But despite all that your husband has to honour his mother as much as he can, and Allaah does not burden any soul beyond its scope. If he can visit her when this son is not present, that is good. If he can keep in touch with her by phone, then let him do so, and explain to her what he feels, this is good as well. If he can get some people to intervene, women and others, who can influence his mother, then let him do so. And he should seek the help of Allaah and make a lot of du’aa’.
The fact that your husband defended himself in court and in front of the police is also good.
If he takes his brother who wronged him to court to stop him from doing more harm, that is permissible, but if he bears it with patience that is better for him.
Trying to get your husband’s mother away from this oppressive brother (as described in the question) is a good idea, so as to remove the means that your brother is using to try to control the whole family.
There is nothing wrong with you stopping visiting her for now, until things settle down, so that you will not be harmed again as happened before, especially since your husband’s mother is not one of your relatives with whom you are enjoined to uphold ties and honour them. If you do not visit her this is not regarded as disobedience or severing of family ties.
Yes, you will be rewarded for visiting her and treating her kindly, if you do that sincerely for the sake of Allaah, and this is part of treating your husband kindly, but it is not obligatory for you to do that, especially since she is badly behaved and has a bad attitude. And Allaah knows best.