When introducing your brother's mother-in-law, you can refer to her as your "sister-in-law's mother." This provides a clear and concise way to identify the relationship without having to repeat "my brother's mother-in-law" every time.
For example, if your brother's name is John, and he is married to Mary, you could say, "This is Mary's mother" or "This is my sister-in-law's mother." Both expressions would help others understand the relationship without confusion.