What is the ruling on a woman uncovering her hair and forearms at her brother's wedding, which is held in a private hall designated for women and Mahrams, knowing that there is a possibility of being photographed with a mobile phone without her knowledge, and is it obligatory for her to wear a jilbab and a headscarf out of fear of this?

A Muslim must safeguard their religion and honor in these corrupt times.

If you are reasonably certain that the women present are religious, moral, and trustworthy, and there are no immoral women among them who might describe your private parts, then there is no harm in displaying your adornments, provided your 'awrah is covered.

However, the position of proper Islamic etiquette requires covering that which, if exposed, would diminish one's integrity, even if it is not 'awrah.

Modesty brings nothing but good, and modesty combined with piety is the capital of the righteous.

If you have any doubt about the trustworthiness or character of the women present, then adhere to covering and modesty.