I don't think the German is the problem here. "We are five brothers" sounds completely unnatural to me as a sentence. As an introduction. "Hello. We are 5 brothers who live in the area and would like to practice our English." it is ok.
However, if only one of the 5 is there, and giving information about his family, it would not be said in this way.
Such direct translations are very misleading and if someone goes to the trouble of learning a language they should go for a useful/usable result and not pidgin. - So, Isch habe fertig!