It's 7ijaB not 7ijaD.
The fact that not all the coverd girls are good, is not an evidence or excuse to denny the hijab as an Islamic sign.
The girl that is good but not covered still have a lack.
The girl that is covered but not good gets her own judgement.
Though reading about Islam, u don't seem to have read enough, your comparison of the irl & her cousin says so. that's not how things go in Islam.
" Let there be no compulsion in religion: Truth stands out clear from Error: whoever rejects evil and believes in Allah hath grasped the most trustworthy hand-hold, that never breaks. And Allah heareth and knoweth all things " [Quran: 2-256]