In Islam you get reward for what you intended, so it's all based on your intentions. Two people can do hajj and one does it to show off and one to earn pleasure of Allah, both will get what they intended for. Same thing here, one who covers by obey Allah as He commands Himself in the His book and for pleasing Allah will get that reward, one who wears for other intentions will get that.
As for mahrams forcing their females to wear proper hijab, this is allowed. They shouldn't be aggressive but try to explain and educate the girl also and not just say "just do it cuz i said so". Anyways, it is a direct command of Allah and not only with the girl be punished for disboey but the mahrams will be punished as well because she is in their care. Once she gets married, then the father and brothers could careless what she does because she is in the care of the Husband and he will be punished on J-Day if she doesn't wear hijab of does evil and he doesn't fix her ways.
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