Praise be to Allah.
The mother has no right to tell her son to divorce his wife just because he is going against his family’s or tribe’s custom by taking a second wife, or because she fears that her daughter’s husbands may follow his example, because plural marriage is something that has been permitted by Allaah and by His Messenger (peace and blessings of Allaah be upon him).
The son does not have to obey his father or mother with regard to divorcing his wife, especially if that is for a reason that is contrary to the aims of sharee’ah, namely to have many children and keep Muslim women chaste and reduce immorality.
It says in Mutaalib Oola’l-Nuha (5/320): A son is not obliged to obey his parents, even if they are of good character, with regard to divorcing his wife, because this is not part of honouring one's parents.
Shaykh al-Islam Ibn Taymiyah was asked about a married man who has children, but his mother dislikes his wife and tells him to divorce her. Is it permissible for him to divorce her? The answer was: It is not permissible for him to divorce her because of what his mother says, rather he should honour his mother, but divorcing his wife is not part of honouring his mother. And Allaah knows best.
Al-Fataawa al-Kubra, 3/331
Your husband has to honour his mother and treat her kindly, whilst keeping you as his wife, because it is not part of honouring his mother to divorce his wife.
This mother should be advised and reminded that it is essential to follow the laws of Islam, and to beware of drawing nigh to sin. She should realize that whether her daughters will get divorced or not, or their husbands will take second wives or not, are matters of the unseen which no one knows except Allaah. Whatever He wills will happen, so there is no point in committing haraam actions and splitting up families.
Our advise to you is to strive your hardest to treat your husband’s mother kindly and to win her love, so that your kind treatment of her will erase from her mind the idea of telling her son to divorce you.
And Allaah knows best.