Probably the same reason that English has so many spelling/pronunciation irregularities; i.e., the pronunciation continued to evolve after the spelling had been standardized. Look at the pronunciation of "ough" in "though", "through", "rough", "cough", "thought", "bough", etc. It used to be pronounced the same in all of these words. Now we just have to memorize which pronunciation goes with which word.
"It just is" I think it's just one of those weird things, that just happen. Like in the English word "Bomb" you don't pronounce the ending "b". It has come to my understanding that no language follows all the rules, and when things do something weird, they just do. So it's best to not question what "just is"
The English is ambiguous without context, or unless stated differently "Does he have any brothers or sisters?" or "Does he have any siblings?"
Is the Russian more clear about possible ambiguity - that using есть here can only mean asking about siblings, and not using is asking about the gender of one (and only one) sibling? I'd think that even in Russian you'd need more context to absolutely clear - or could ask it in a better way.
I'm English and it doesn't sound wrong to me. There are places where we don't have to use inversion to form a question. Here it could be that it's known that he has a sibling, but we're not sure if they're male or female. "He has a brother or a sister?" should be accepted.
Ah, sorry, I didn't think about this meaning. However, the Russian sentence focuses on whether he HAS any sibling, so the correct translation is "Does he have a brother or a sister?"
For "He has a brother or a sister?" (implying he surely has someone), you'd say "У него брат или сестра?", omitting "есть".
I feel that the meaning here depends more on the intonation of the speaker rather than the inclusion or omission of "есть".
If you were asking if they had any siblings, you'd raise your intonation for the "есть брат или сестра" part of the sentence, whereas if you knew they have a sibling but don't know their gender, you're voice would go up at "брат" (the first option) and then back down for the rest of the question. It's pretty similar to English intonation changes.
This question asks if he has any sibling(s). There is no word for "sibling" in Russian, so we say "брат или сестра". You can also ask "Does he have brothers or sisters?" - "У него есть братья или сёстры?" And yes, you can answer "У него два брата и (одна) сестра". You don't really need "есть" in the answer, and you also don't have to say "одна" (note it's feminine).
Why is он inappropriate to use for the sentence? Why cant it be "у он есть". Why are there so many different words to convey he, she, they, we, etc.
genitive case goes with у. it shows possession.
Russian does not describe possession with a specific verb as in English, but with the expression "у Х есть У", which means "at/on somebody there is something/somebody". Example:
- у него есть врат.
- On/at him, there is a brother. (literal translation)
- He has a brother. (not literal translation)
In Russian, /v/ is often realized with weak friction [v̞] or even as approximant [ʋ], particularly in spontaneous speech. You're hearing the latter. You can hear its pronunciation in https://en.m.wikipedia.org/wiki/Voiced_labiodental_approximant .