Brother-in-law
You aren't related by blood, however, he would be referred to as Uncle.
Your mother's husband's sister is your mother's sister-in-law. Her sister-in-law's husband is not related to your mother, but is her husband's brother-in-law.The most common ways of referring to him would be "my sister-in-law's husband" or "my husband's brother-in-law."
He's called your uncle becasue your mother's sisters are your aunts and the people married to them are your uncles (assuming that they're men).
Your mother's sister's husband's sister is your aunt's sister-in-law, but is not related to you. Here's why: Your mother's sister is your aunt. The sister of a woman's husband would be her sister-in-law. In this case the woman is your aunt. So your mother's sister's husband's sister would be your aunt's sister-in-law.
Your mother's sister's husband - would be your uncle.
Your mother's brother is your uncle. Your father is his brother-in-law but he could also have other brothers-in-law. These would be men married to his other sisters, or the brothers of his wife. If they are men who married his other sisters, they are your uncles. If they are brothers of his wife, they are not related to you.
In most families she would still be referred to as your mother-in-law. If you remarry, you would introduce her as your first husband's mother.
Your mother's sister's daughter is your first cousin. Your first cousin is not related to you. You would call him by name, or refer to him as "my cousin's husband."
Your mother's only sister would be would be your aunt. Your aunt's husband would be your uncle. Your uncle's sister in law would be your aunt's sister who is your mom. You were introduced to your mom.
No because you are not involved in the marriage. Your husband's or wife's sister would be your sister-in-law. Your son's mother in law is not related to you in any way.
Your are related to some of your first cousin's aunts and not to others. The aunts of your first cousin to whom you may be related are your mother and her sisters or your father's sisters. One of the parents of your first cousin isn't related to you, but married into the family. Anyone of your cousins aunts who are related to that person would not be related to you.
Your mothers husband would be your step father and you would be a step son or step daughter.