Once a couple is married the spouse’s parents become a "mother-in-law and father-in-law the other spouse. The spouse’s siblings are the brother-in-law or sister-in-law of the other spouse. But we do not have a designation for the relationship between spouses of siblings. That is to say, if Bill is married to Betty and her sister Kate is married to Kevin, Bill and Kevin are not technically brothers-in-law. They are men who are married to sisters. While each husband is the brother-in-law of the other sister, they are not given a designation in this country. Though in a family where everyone is on good terms Bill and Kevin are likely to refer to one another as “my brother-in-law.” If things are less cordial it just may be “my wife’s sister’s deadbeat husband.”
Other languages sometime acknowledge relationships that we don’t in English. There are three Yiddish terms that are particularly useful in the “in-laws” category.
The first is “Mekhuteneste,” which means “My daughter-in-law’s mother” or "my son-in-law’s mother.” Or the mother of my child’s spouse. Mekhutn is the father of my child’s spouse. That is to say my daughter-in-law’s father, or my son-in-law’s father. The last is Mekhutonim, which are collectively the parents of you child’s spouse. This acknowledges a relationship that often must be dealt with, that English doesn’t even have a word for.
Another relationship is forged by law when people who have had children with another partner marry each other. The “step” relationship is an interesting one. If one of your parents is married a person who is not your other biological parent they are a “stepparent.”
Miriam-Webster says the earliest know English use of stepparent is from 1840. Wikipedia, on the other hand, places the origin much further back in time. “The earliest recorded use of the prefix step-, in the form steop-, is from an 8th-century glossary of Latin-Old English words meaning, "orphan". Steopsunu is given for the Latin word filiaster and steopmoder for nouerca. Similar words recorded later in Old English include stepbairn, stepchild and stepfather. The words are used to denote a connection resulting from the remarriage of a widowed parent and are related to the word ástíeped meaning bereaved, with stepbairn and stepchild occasionally used simply as synonyms for orphan.”
While it does not have to be the case, it is interesting that some sense of loss or grief is wrapped up in the term “Stepparent.” After all either death or divorce must separate the original set of parents to allow for the “step-relationship to exist. Either way there was some kind of loss involved.
If your non-biological parent has children they are your stepsiblings. On the other hand should your parent and their spouse have a child together, that child is a half sibling. Meaning you share the genes of one parent, but not both.
Many languages differentiate grandparents based on which parent they are the parents of. While we say grandmother or grandfather without usually commenting if that relationship is paternal or maternal, the Swedes are more specific; Mormor means mother’s mother. Farmor means father’s mother. Farfar means father’s father and Morfar means mother’s father.
While many English words come from Latin origins, we long ago dropped specifying if an uncle is related to our mother or father. Avunculus denoted a mother’s brother, while patruus indicated a father’s brother.
I guess how we define our familial relations is less important than just trying to stand each other enough to spend a long holiday weekend together?