-im and -ot may be standard Hebrew plural endings, but they often change the root, and when I can't figure out how I would change it back, it's often a sign (to me) that the word is an import. But not always, my Hebrew is native but beyond rusty.

The words in the linked article (choten and chotenet, cham and chamot) aren't the ones we're discussing here, and in the article, it only points to the word mechatnim מחתנים as a verb meaning "marry off". "When the Smiths marry off their daughter."

Here are some more: http://en.wiktionary.org/wiki/co-father-in-law