What is the ruling on my brother evading taxes and selling clothes in a grocery store for less than the market price? And is it permissible for me to buy from him?

The brother's evasion of taxes depends on the ruling regarding the tax itself. If it is permissible, then it is not allowed to evade it. If it is forbidden, then it is allowed to evade it. As for engaging in trade without a license, its ruling has been clarified in a previous fatwa. The brother selling clothes in his shop specialized in foodstuffs is permissible, and he may sell them at any price, and it is permissible for you to buy from him.