I'm not so sure about "in-laws" for cousins.
I may be an informal term used by some people but I don't believe it has any real standing. As far as I am aware, the only formal use of "in-law" is for Mother/Father-in-law, Son/Daughter-in-law and Brother/Sister-in-law. Even this last example is often misused. A spouse of your Brother/Sister-in-law is not also your Sister/Brother-in-law, but is often cited as such. Your Brother-in-law can be either your sister's husband or your wife's brother but it doesn't extend beyond that.