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.