My sister’s kids and my wife’s sister’s kids are my nieces and nephews. I am an uncle to each of them.
In one case I am a blood relative (0th cousin once removed) and in the other case unrelated (although in reality a distant cousin).
Is there a distinction in English or other languages, or is “uncle” the universal term?
Uncle and Aunt are properly your parent’s male and female siblings on both sides. This is often extended as a courtesy title to other relatives and friends of one’s parents. I had several, who were not related in any way. One could also use the title for great aunts and uncles - one’s grandparents’ siblings and even beyond.
Chinese has distinct words for a wide array of family members.
For example, “aunt” in Cantonese
Even in English, it’s not uncommon to hear someone refer to an uncle or aunt by marriage.
My nephew (my older sister’s son) actually defined this for our family: “If they are around my parent’s age, they are my aunt/uncle. If they are my age or younger, they are cousins.”
My family is pretty extensive and many of them are not actually related to us, but are considered family none the less
I don’t speak it, but I hang around in circles that speak Hindi, and there’s quite a lot of specificity regarding aunts and uncles and other familiar relationships.
In Spanish both are tío/tía, but you could be more specific with “tío/tía carnal” (flesh uncle/aunt) for mom/dad’s sibling.
We also use tío/tía for our friends’ parents.
And while tío/tía postizo/a (“attached” or “false” aunt/uncle; think hairpiece) is impolite and unlikely to be found in writing, it’s the equivalent of the English uncle/aunt by marriage.
There’s some relations which in English are cousins Nth removed and in Spanish are types of uncles/aunts: parents’ cousin = cousin once removed (there is no indication if up or down the tree, is it?) = tío segundo.
And of course there’s your tíos abuelos (great uncles) and your tíos bisabuelos (great great uncles) and your tíos tatarabuelos (great great great uncles)… I even know several of my tíos abuelos segundos, cousins of my grandparents, and of my tíos bisabuelos segundos, cousins of my great grandparents 
The English words “uncle” and “aunt” each covers five relationships in Bengali—
Jyethamoshai/Jethu— father’s elder brother
— his wife: Jethima/Jethi
Kaka— father’s younger brother
— his wife: Kakima/Kaki
Pishima/Pishi— father’s sister
— her husband: Pishemoshai/Pisho
Mama— mother’s brother
— his wife: Mamima/Mami
Mashima/Mashi— mother’s sister
— her husband: Meshomoshai/Mesho
So the question then becomes -
Is the english aunt / uncle specific enough? As demonstrated by other languages, there are different words for different relationships - which way is better?
There’s also “tía política / tío político” = uncle / aunt by marriage.
I married into a Danish family, Danish is bizarrely specific about who’s who. From Wikipedia:
Aunt and uncle are perfectly adequate in English (for us Murkins, anyway. I won’t presume to speak for our cousins across the pond in Old Blighty). This includes uncles and aunts by marriage. Arguably, even by former marriage.
For instance, my late mother-in-law’s sister was briefly married to Alan Jay Lerner. Even though he married Fred McMurray’s wife from the Absent-Minded Professor before my wife was even born, in my household, we still refer to him as “Uncle Alan.”
Seven words for aunt! Four for the {older,younger} sister of your {father,mother} and another three for the wife of an uncle. (Only three because the older/younger information is suppressed in the case of mother’s sister-in-law.)
Ignoring obsolescent terms, Thai makes do with a mere three words for aunt. Father’s older sister (or older brother’s wife) and mother’s older sister (or older brother’s wife) all share the same word (ป้า). Father’s younger sibling (or younger sibling’s spouse) all share a second word (อา). Mother’s younger sibling (or younger sibling’s spouse) all share the third word (น้า).
Similarly there are three words for uncle: (ลุง) and two of the words just described for aunt: when the aunt/uncle is your parent’s younger sibling, that person’s gender is not implied by the kinship word; instead the gender of the relevant parent determines the word!
The older/younger relation is determined by the blood relative. My wife is younger than her sister who is younger than me, but my nephews call me by the word for younger aunt/uncle.
Since all languages can describe each relationship with precision when necessary, there’s no “better” or “worse,” just different. And if you’re rationing syllables and prize specificity, perhaps “more efficient” or “less efficient.”
Personally, I always have to specify “my good aunt” and “my evil aunt,” and I’m not aware of a language on earth that has the necessary kinship terms.
Languages evolve to fit the customs of the people. In the languages are more specific there are cultural reasons for this. It’s important in Chinese to know if the aunt or uncle is older or younger than the parent. This was not as important in England when the words developed.
English has as many kinship terms as it needs. If a language differentiates, say, between cross and parallel cousins (a cross cousin is the child of your father’s sister or mother’s brother) it is most likely because cross cousins are not subject to incest taboos (in fact are often most favored mates) while parallel cousins are.
My late colleague Joachim Lambek wrote a number of papers on kinship terms in various languages (always using native informants). Unfortunately, they appear in the Journal of Theoretical Linguistics and are behind a paywall.
I’m not sure that this is the question that it becomes.
I recall reading recently that the words we use affects the way we think about things. And it certainly seems to me in societies that have more precise kinship terms tend to treat the differently-named relationships as different kinds of relationships.
In Anglo-American culture, uncle/aunt/cousin are very broad terms, especially cousin. I recall being surprised as a child to learn that in English, “cousin” could mean someone in a different generation. Before that, I had been using it only for relatives in my generation.
I’ve always found it odd that English doesn’t have a collective term for aunts & uncles. I mean, your father & mother are your parents, your brothers & sisters are your siblings, your sons & daughters are your children, but your aunts & uncles are… uh… aunts & uncles.
In Arabic the word indicates whether the aunt or uncle is the sibling of your mother vs. your father. I’m not sure if the words are atomic or really just phrases that translate literally as “the sister of my father,” etc. I will check with my Arabic-speaking wife.
Adding the correct spellings in Bengali —
জ্যাঠামশাই / জেঠু,
জেঠিমা / জেঠি
কাকা
কাকিমা / কাকি
পিসীমা / পিসি
পিসেমশাই / পিশো
মামা
মামিমা / মামী
মাসিমা / মাসি
মেসোমশাই / মেসো