The sister to a husband becomes the wife's sister-in-law. If the sister in-law was to marry a woman, would she also become the wife's sister-in-law? If so can you keep calling the family of your in-law's spouse in-laws as well?
For example,
Would Amy use the term "in-law" to describe Arisa and her family?
According to Wikipedia (although no sources are cited):
Your sister-in-law is either the sister of your spouse, or the wife of your sibling. So Nanoha is Chrono's sister-in-law, and Fate is Amy's sister-in-law.
Shinobu is the wife of the brother of the sister of the spouse of the sister of Chrono. This doesn't fit either definition. I think she's Chrono's sister-in-law's sister-in-law, and Amy's sister-in-law's sister-in-law's sister-in-law, but we don't have a specific term for this relationship in English (there are some societies with much more elaborate naming schemes for familial relationship, I wouldn't be surprised if they have terms for this). And Arisa is then another step removed.
However, sometimes people use the general term in-laws to refer to everyone they become related to by marriage. So they might then refer to all these additions to the family after marriage as their in-laws, but I think this is usually reserved just for close relatives. It's usually just parents and siblings, you don't generally hear people referring to cousins-in-law, for instance. When the distant relatives are related only through several steps of marriage and siblings, they're not likely to be included unless you actually socialize with them much.
In many cases, we simply ignore the marriage aspect: when you marry someone, their neices and nephews become yours as well, we don't say neice-in-law or nephew-in-law.
As this famous scene from Spaceballs puts it:
Dark Helmet: "Lone Starr - there is something I must tell you. I am your father's brother's nephew's cousin's former roommate."
Lone Starr: "And what does that make us?"
Dark Helmet: "Absolutely nothing, which is what you are about to become."
An in-law technically only works to one degree: My wife's siblings are my in-laws and vie versa - but even my parents and her parents are not related in any fashion.
In more common parlance one can speak of the "in-laws" less specifically, to denote a tie by marriage, but going by the specific definition, it only applies to the immediate family of your spouse.
From here:
The earliest recorded use of the phrase is in brother-in-law (13c.); the law is Canon Law, which defines degrees of relationship within which marriage is prohibited.
The purpose of the term was to exclude non-marriable individuals. A man cannot marry his in-law or vice versa - but there is nothing to prevent a degree of marriage in this instance.
