"Tì! Tapadh leat a bhràthair!"
Translation:Tea! Thank you, brother!
Learners and younger speakers generally pronounce both bh and mh as /v/. However, they were different traditionally, and you may still hear this. Bh is made with both lips, like Spanish b and v (which are the same). This means it is between /b/ and /v/. That in turn means that if you are not used to it you may interpret it as a /b/. If you listen carefully you should hear that it is different from a /b/ because the airflow is never entirely stopped. (This is what linguists call a bilabial voiced fricative, written /β/.)
The corresponding pronunciation of mh is a nasalized /v/, written /ṽ/. This is like a /v/ while holding your nose or with a heavy cold. However this does not cause confusion as there is nothing to confuse it with.
But this does mean that if you hear /ṽ/ or /β/ then you can tell whether it is a mh or bh which can sometimes be helpful. D
The interesting point about this question you ask is that all language will vary a bit from family to family, and this particular issue is, by definition, something you would only hear in the family, so most people would not know what goes on in another family. This would lead to great diversity in the language, in the same way as English speakers in different families used a wide range of different ways of addressing their parents.
But I would guess that if your brother is still a child living at home there would be no question that he would be thu. If he were much older than you and had his own family then you as a child would clearly address him as as in the same way as you would an uncle — with sibh. Before the days of contraception, and with people marrying very young it was far more common than it is now for brothers to be anything up to 25 years older than you, meaning your brother could easily be older than your uncle. But if you had a range of siblings, it would be anyone's guess where you would draw the line.
Yes. You may have read the other comments, but not understood because you are missing the basic issue. There are two parts.
Firstly, leat is singular – when you are talking to one person. Leibh is plural.
But secondly, we use the plural as 'polite singular'. This is exactly the same as French, Dutch or Welsh. If you are not familiar with any of these it is a bit confusing, and that is what the discussion on this page is about – would you ever need to use the polite form with your brother if he were much older than you.