The mother-in-law of your daughter is commonly referred to as your "daughter's mother-in-law." Similarly, the father-in-law of your daughter is referred to as your "daughter's father-in-law."
For example:
- If your daughter's husband is named John, then John's mother would be your daughter's mother-in-law.
- If your daughter's husband is named John, then John's father would be your daughter's father-in-law.
It's worth noting that family relationships and titles can vary depending on cultural practices and individual preferences. In some cultures or families, more specific or different titles may be used to address in-laws. However, "mother-in-law" and "father-in-law" are the most commonly used terms in English to refer to these relationships.