Where I am from, auntie is a familiar form of aunt. We put -y or -ie on many words when speaking familiarly or to small children. So we have doggie and kitty and such as words we use with children, but never in a more formal setting. Is antaidh also strictly familiar, or is it used in more formal situations, too?
Like "tìoraidh" for "cheerio", it seems "antaidh" is a Gaelic way of saying "auntie". I believe the formal word is "piuthar-athair" or "puithar-màthair" depending on which parent is related and it mattered in the conversation. But to use it you would need to know the relationship, so "antaidh" is easier to use here.