You can say "너희 오빠".
"너희 ~" means "~ of you and your family" or "~ of you and your fellows" or "~ of you your surroundings".
너희 오빠한테 사달라고 물어봐도 되?
Can you ask your brother(of your family) to buy it?
or you can simply say "오빠", as the listener knows it means his/her own brother.
오빠한테 사달라고 물어봐도 되?
Can you ask your brother to buy it?
i want to add one more thing.
"돼" is "되+어" so, it is correct using "되" in this sentence.
hope you can understand^^