Is severing ties with the husband's mother permissible or forbidden in the event that she causes constant problems with the husband?

You must maintain ties with your maternal aunt despite her mistreatment; there is no humiliation in this, but rather elevation for you. Your continuous kindness may lead to a change in her attitude. Prudently explain to her the prohibition of severing ties, using evidence from the Quran and Sunnah. Remember that she is not only your maternal aunt but also your husband's mother, and your husband's happiness is tied to your relationship, so never allow a severing of ties to occur.