Is it okay to teach C++ to my 7 year old brother?
I want to tack on to Buffy's answer by focusing on one line from your question: "I want him to learn C++,Java,Html" (emphasis added).

The bigger question should be "What does he want to learn?" 

Part of the beautiful challenge that is teaching is getting students to want to learn what we want to teach them. What has your brother shown interest in? What has he told you that he wants to learn? What are his hobbies? How could something about those hobbies be expressed/constructed in a program? Above all else, where do his interests align with what you can teach him?

I'd encourage you to read this thread about starting students in Python, this thread on engagement with computing, this thread about programming languages for teenagers (shout out to Processing!), and our recent HNQ about the age at which students should begin learning programming.

Make it fun and engaging for him. Have him build a game, one that he himself would want to play. The language skills will come with time (and intellectual maturity). What you can do now is two-fold: pique his interest for the future and teach him the essential concepts of computational thinking that underpin programming in any language.