well.. to begin with.. you said "c'est tres mignon et mechant" .. first you are talkign about your brother.. you cant use the term "it" .. see it doesn make sense in english even .. so u should use "he is" instead.. which is il est tres mignon et mechant.. secondly.. your grammar .. il a des cheveux noirs, des yeux marron " umm.. you should use the term "les" instead of des.. and yeah you cant say "J'aime beaucoup.." if you wanna say you love your brother should say.. something like "J'adore mon frere beaucoup.."
hmm guess thats it for now.. was few more.. but tehse were the msot distinct.. have to go study physics actually.. hehe.. but inshaAllah ill post few essay stuff when i can soon. till then Take care n dont worry