MIL sabotaging family relationships

My husband was raised by what I believe is a narcissistic mother. For example, she speaks negatively about her children, with the exception of her eldest. She showed no interest in getting to know me when my husband and I were first dating, and upon marrying, she only wanted to speak with me negatively about my husband (really awful untrue things). She shows next to no interest in her grandkids. It is my belief that she has sabotaged my husband, and now my and our children’s, relationship with his extended family. Does anyone have experience with this? How do you recommend dealing with it? If there are valid extended family relationships, I would like to have those people in our lives but they also seem oblivious to her manipulation. Any and all advice or insight is greatly appreciated!