"Frere, je veux vous parler" wasn't accepted. I get that maybe it's too formal for a brother - but is it grammatically wrong?
Surely, "Frère, je veux parler à toi" can work, too? That is literally, "Brother, I want to talk to you."
This is frustrating, since multiple translations are often possible.
"Frère, je veux parler avec tu" doesn't work. Why does it have to be toi?
tu is a subject, toi is an object. It's exactly like the difference between "I" and "me" in english. You wouldn't say "Speak with I" , you'd say "speak with me".
The statement says to "... talk TO you." not "... talk with you" - so why wouldn't "parler à toi" be correct.