Consider this phrase
Today I toured around the city with an aunt, uncle and cousins who came to visit me.
Is there a shorter word/expression that unambiguously refers to my aunt, uncle and cousins?
I am aware of relatives, but this is not unambiguous. So, if I say relatives, someone might feel the need to ask for further clarification, for instance, if it was my nuclear family who came to visit me.
No.
There isn't actually any suitable idiomatic word/phrase that's pretty common and casual and can be used in day-to-day conversations and writings. None of the other answers address this. Your context gives the impression that it's a normal piece of writing. Second-degree relatives, extended family, whatever you say— they're technical terms and don't even specify that it's your aunt, uncle, and cousins and not your grandparents. Most people will not get you easily and it'll probably make you sound awkward or nerdy.
The word you want is just too too specific. Just say it—my aunt, uncle, and cousins. What's wrong with that? It doesn't do to be lazy and sloppy in writing.
In fact, people will like hearing more about them. Describe them better if you can. It'll give you a topic to talk on, triggering conversations. Short, concise, to-the-point writing is for academia.
X: You know, my aunt, uncle, and my cousins came to see me yesterday. I toured around the city with them.
Y: Oh, cool! It must have been exciting. Did you have fun?
X: Yes, my cousins are such cuties. Amanda's 5 and she...
you might consider — my aunt and her family or my aunt's family. Personally, I like the former better:
Today I toured around the city with my aunt and her family who came to visit me.
An aunt sounds odd to me. You don't need to hint that you have more aunts and it's just one of them. It's your aunt and not somebody else's — that's more important to focus on.
Alternately, if your uncle's the direct relative and your aunt his wife, then say "my uncle's family" instead.
It's still somewhat ambiguous, but
"Extended Family"
might be a better phrase, or possibly
"Some of my Extended Family"
to indicate it's not all of them.
Edit: To improve this answer I'm also including some dictionary entries
http://www.dictionary.com/browse/extended-family
- (loosely) one's family conceived of as including aunts, uncles, cousins, in-laws, and sometimes close friends and colleagues.
https://www.collinsdictionary.com/us/dictionary/english/extended-family
An extended family is a family group that includes relatives such as uncles, aunts, and grandparents, as well as parents, children, and brothers and sisters.
A better expressions would be "My aunt/uncle and her/his family" which implies my aunt or uncle and their immediate family. You could use "mother's family" or "father's family", although this could refer to their parent, grandparents etc..
Edit: I use this myself - when my wife's brother comes to England from the US with his wife and children, I say that "We spent the day with my brother-in-law and his family".
If the aunt is your blood relative of your father, then, "My father's sister's family", gets us to four words and reduces ambiguity by turning the sentence into a directed graph of your family tree. If you really want short, "With Kin." is about as short as you can get, but you compromise on precision.
Technically your aunt is your first cousin-once-removed, so "With cousins" is a tie for short, (in words if not letters) but again fails on the ambiguity front.
Its up to you to balance precision and accuracy, depending on your audience.
