על המילה מחותן

You asked about the word "mechutan". It is one of the so many words in Yiddish which are actually Hebrew. In Hebrew itself it is a comparatively new word, not attested before the 16-th century. Its root, though, is old, and it is also the root of the words "chatan" – 'groom' and "chatuna" – 'marriage', 'wedding'. So literally it means 'connected by marriage', namely 'relative by marriage'.

I don't know of an exact parallel in English. I think in Hungarian they call them "anyatárs, apatárs".

מה דעתך על אלה?

ה"א השאלה בלשון ימינו להתחתן בתנ"ך – לא מה שחשבתם! (פרשת ואתחנן) "בעל" נגזר מ"בעלות"? על חלופות ל"בעלי" בתוככי ירושלים