Normally, no. In a sentence like this, भाई is an unmarked plural meaning "brothers". In the oblique case, however, meaning when it is the object of a postposition, then all masculine plural nouns regardless of whether they are marked or unmarked take ओं, giving us भाइयों.
So: जूलिया के भाई means "Julia's brothers", but मैं मेरे भाइयों के साथ खा रहा हूँ means "I am eating with my brothers"
That's pretty complicated. People often use wrong hindi colloquially. But for example when you say भाइयों और बहनों when you are addressing a crowd, is that also wrong?
And how would you say julia's brother? जूलिया का भाई ?
Yes, that is how you'd say Julia's brother. When addressing someone, words also take the oblique case. Thus, भाइयों और बेहेनों!
Yep! Although a tiny tip that in your example it should be मैं अपने भाइयों, not मेरे
The plural of भाई ( mas. sing.) is भाई only. While भाइयों is Vocative plural., vocative singular being भाई.
Has it somewhere already mentioned when you have to say "ka", "ke", or "ki" ? What I found out from google translator: Peter's brother: ka Peter's sister: ki Julia's brother: ke Julia's sister: ki
Is there a general rule for this?