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It must be a dwarven quirk. A reminder of their mother, his sister while on the road, far from home, far from hearth, far from comfort.
If there was one thing a hobbit understood, it was comfort. And the reminder of someone waiting for you at home was comforting indeed.
And perhaps the melancholy intent with which Bilbo wondered at the phrase had to do with the reminder of the empty Bag End behind him.
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But Dwalin would say it too. And when Balin ever called them anything else it was 'Thorin's heirs', so maybe it was the best translation from dwarvish?
Was the dwarves' tongue so specific a language that by the term one addressed a relative it covered not only how they were related but through which branch and individuals?
That would stir up a mess during festivals and birthdays in the Shire: 'Yes, mother's brother, I have seen the lovely couple, my father's brother's son and my mother's sister's husband's brother's wife's daughter.' It was much easier to just use names!
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But then, Thorin did call them by their names, and not just when speaking to one rather than the other.
And Ori certainly chided them with their names. But then Ori was barely older than Fili, if Bilbo was any judge of dwarven aging. So perhaps it was a tag-along from their kit days.
Nori and Bofur each used 'Thorin's sister-sons' or 'Thorin's lads', so must one recognize the family of the speaker? Bofur also used 'my brother' and 'my cousin' more than Bombur of Bifur's names. And Nori could somehow use 'my brother' to refer to each of his brothers but allow his tone to fully convey whether he meant Dori or Ori.
Dori could do the same with Nori or Ori when speaking of the two, but he always used their names when speaking to them.
Oin called Gloin 'my brother' more than his own name, and nearly the remainder of the company was 'cousin' unless they were his patient. At those moments, he always used their names.
Gloin would constantly sing the praises of 'my wife' usually with words like 'gem', 'lovely', and such, and when it came to Gimli it was 'my son', 'my wee lad, Gimli'. Bilbo thought he could understand the phrasing, as neither of those two were actually present.
Bilbo had no idea what Bífur said when he spoke, but then Bombur was usually silent and gave Bilbo no frame of reference either.
Even Gandalf would use the accursed phrase when referring to Thorin's nephews. But why?
~~~~~*~~~~~*~~~~~
It wasn't until their reprieve at Beorn's that Bilbo dared ask. He did so as the company was clambering across the hay to claim sleeping spaces. It was admittedly the softest hay he'd ever tried to sleep on, so he held high hopes for his night in Beorn's nice, warm, and safe home.
"Thorin?" Bilbo asked, emboldened by Thorin's apology on the carrock. "When everyone refers to your nephews, why do they say sister-sons instead of nephews?"
Thorin glanced sharply at Bilbo before glancing over at the two youngest dwarves members of their company. "Because, Master Baggins, they are my heirs."
"Bilbo, none of this 'Master Baggins' nonsense, Thorin. And, well. I knew that, but why not just say nephews though? It is the Westron term. I mean, yes, sister-sons does get more specific in their parentage, but I had noticed that everyone else bears their father's names. So why are Fili and Kílí named through your sister?"
Thorin peered at the hobbit for a long moment before speaking again, slowly, "They are my heirs, Bilbo, because they are my sister's children. If my brother Frerin had lived to sire children, I would have been an enthusiastically doting uncle, and pleased to see his line go on, but his children could never have been my heirs."
Bilbo scrunched up his nose. "Well, but... Why ever not?"
Thorin hummed. "It is an old tradition. Either the king's children, or the children of a sister to the king, can inherit. It is the only way to guarantee that those who inherit are true heirs of Durin." Seeing the perplexed look on Bilbo's face, Thorin continued, "If I were to wed a dam, and that dam decided to lie with another dwarf, the child she bore might not be mine. I would hope to never doubt my spouse's fidelity, but in matters of succession of this stature, it's significance cannot be set aside. The same goes for my brother. My sister's sons, however, will always be kin of my line, no matter their father; because they were absolutely the children of my sibling, with zero doubts."
"Oh."
"So, Bilbo, I call them-and so do the rest of the company-my 'sister-sons' because it is their due to be my heirs, as her children. If I had ever had children of my own, I could still have named Dis's sons my heirs over my own get, with no fuss."
"Oh," Bilbo said again. "I see." He didn't say anything more for a few minutes, beforeasking, "Should I call them that too, then? It's just, I've been saying 'nephews' all this time and-"
"Peace, Bilbo. I think Fili and Kílí enjoy the novelty. Besides, as a Hobbit, we could hardly expect you to follow every dwarven custom, now could we?"