we don't need của in this case?
You can omit it in this case.
Special rules for familial terms seems to be a common feature of many languages.
Why isn't "older brother" accepted?
I'm sorry, "anh" here is pronounced like "ai" and my native-speaking Vietnamese friends definitely do not say it that way.
Sorry, I hear clearly "anh"
Your friends probably speak Southern Vietnamese, where "anh" it is pronounced the same as "an". The Duolingo course uses Northern pronunciation.
I suggest that "That is my big brother" should also be accepted. That is a much more common way of referring to your older brother in American English
I understand that "anh" on its own is elder brother. What does Trai do in this sentence?
Older is the same as elder!!
big brother shoild be accepted. come on now DL....