We've tried to explain to him that there's no word in English for "unmarried life partner of my father." I am definitely family, but I don't have a category.
em's right! There ARE no cool words for the modern day complexities of relationship.
There are some legalese words which sound about as tender and loving as rubber floor mats in cars. Words like "de facto" and "common law wife/husband" have about as much appeal as cold porridge or wet blankets.
I guess you could call em, Steve's "concubine" (yes the definition of this being "A woman who cohabits with a man without being married to him" - Macquarie Dictionary Australia - go look it up!)... but this word has other subtle connotations I'm sure would make em choke on her Coconuff Fudge and I really like her far too much to have her die from such a thing! ;)
Triston is on to something!
Modern day love needs new definitive terms that describe the differences succinctly but also tenderly. Marriage, un-marriage, de-facto, gay and lesbian unions, polyamory unions and committed friendships are a whole new ballgame for many people needing new paradigms and agreements on how to tenderly name the relationships that grow from these complex arrangements.
The thing Triston has done though is to bring a powerful sense of family to em's role in his life. She is "my Michelle". This simple, definitive and somewhat possessive term of endearment puts em at a central and integral place in his life. It describes why he loves her and how important she is to him.
Love spawns complexity. It grows roots and branches among people in strange and often confusing ways. The boundaries to the traditional understandings of mere "Marriage" and "Union" has and is becoming more and more blurred.
The most important thing though, is that whatever new words we find to describe particular relationships - that we also acknowledge and recognise that those relationships are about LOVE before anything else.