Is a woman sinful if she uncovers her shins in front of other women? And should she object to those who do so, knowing that the adopted opinion on your website is that it is permissible for a woman to display what is customarily shown in front of her peers?

The opinion of the majority of scholars regarding a woman's 'awrah in front of another woman is what is between the navel and the knee. There is no blame on whoever adopts this opinion. However, it is more cautious for a woman not to expose to another woman anything other than what is commonly apparent. Indeed, enjoining piety and caution is good.