Three members of my extended family celebrate their birthday today. They are not genetically related to each other (there is one ‘in-law’ between each of them) and only one is genetically related to me. The first, chronologically, is my brother-in-law, my oldest sister’s husband. The second is his sister-in-law – not either of the two who are my sisters, but his brother’s wife. There is no term in English for my relationship to her; she is ‘my brother-in-law’s sister-in-law’. I have met her occasionally, and am Facebook friends with her, but it is perfectly possible never to meet one’s brother-in-law’s sister-in-law. I have not met my youngest sister’s husband’s brother’s wife (and they live in the same part of the same city as me). The third is my niece, my second sister’s daughter. She is that brother-in-law’s wife’s niece, niece-in-law or maybe just niece; he is her aunt’s husband, uncle-in-law or just uncle. (She calls him ‘Uncle [name]’ just as she calls me ‘Uncle [name]’.) There is no term in English for her relationship with that other relative: they are ‘uncle-in-law’s sister-in-law’ and ‘brother-in-law’s niece-in-law’ respectively. Maybe some languages have a kinship term for ‘female relative one generation older (other than mother or aunt’ (or simply use the word ‘aunt’) and ‘female relative one generation younger, other than daughter or niece’ (or simply use the word ‘niece’). These three members of my extended family have been in the same place twice, as far as I know: my oldest nephew’s and my oldest niece’s weddings (the son and daughter, the cousins and the nephew- and niece-in-law of those three people). My nephew’s wedding was a few days before those people’s collective birthday, so the next day many of the same people gathered again for his birthday. We got those three birthday people together for, as far as I know, the only photo of the three of them (though they might all be in an ‘extended family’ wedding photo).
So, my oldest nephew and niece are married (to other people, of course!). I usually refer to my niece’s husband as my nephew-in-law, because I have a less close relationship with him, but I usually refer to my nephew’s wife as my niece, because I have a closer relationship with her. But it does cause confusion. Sixteen months ago I told a colleague that my nephew and niece had just had a baby. She asked for clarification!
How far does one’s ‘extended family’ extend? I know who my brother-in-law’s brother-in-law’s [second] cousin’s step-mother is, but I wouldn’t call her ‘family’. (By the way, I’ve got two or three sets of brothers-in-law: my sisters’ husbands (three sisters, one husband each), who are ‘my Australian brothers-in-law’, my wife’s brothers (one wife, two brothers), who are ‘my Korean brothers-in-law’, and maybe my wife’s sisters’ husbands (one wife, three sisters, one husband each).