Is the wife sinful for not speaking to her husband's brother, with whom she had a dispute? And what is the ruling on her attire and adornment in front of her husband's brothers?

A woman must exercise extreme caution in her interactions with her husband's brothers and male relatives, as they are non-mahram to her. She must observe the full legal hijab in front of them, and it is not permissible for her to be in seclusion with any of them, or to travel with them, or to speak with them except out of necessity and to the extent of that necessity, while observing Islamic etiquette.

As for shunning the husband's brother, if he is known for his hostility and his insults have been frequent, and if shunning him preserves the woman's religious and worldly well-being, then there is no harm in boycotting him. This boycott would be for the sake of Allah and in Allah, as a recompense for his harm to Muslims and his ill-manners towards them. And perhaps a beautiful shunning is better than harmful mixing.