Yeah sorry a lot of these sentences were not only wrong but also inconsistent which is worse xD
So, do people omit articles or possessive pronouns when talking about aunts and uncles the same way they do when talking about moms and dads? E.g. "I went there with mom/dad" -> "I went there with uncle/auntie"? "Dad is with mom" -> "Uncle is with auntie"? If it's correct, then it should be accepted. Otherwise the only thing should be "An/the uncle with an/the auntie". Since Ukrainian does not have articles, either combination is OK.
"Brother with sister" -> again, do people omit possessives in English with brothers and sisters? Doesn't seem so to me. But I'm not sure, so I'm asking to double-check :')
"Брат ... із сестрою" can mean "a brother ... with a sister" (general case), "the brother ... with the sister" (assume he has many sisters and he is eating with a specific one), "the brother ... with a sister" (assume he has many and we don't know which one it is), "the brother ... with his sister" (the actual most common meaning in Ukrainian here, we omit possessives)
I don't think it's common. I don't know any people who refer to their siblings just as брат and сестра. They would either say the name or add мій/моя.
The nun meaning would work in Ukrainian too :') Didn't think of that... Should I add all the "brother is eating with sister" versions now then? :D