Should the husband's brothers, who trade in alcohol, gamble, and deny the Day of Judgment, be boycotted, taking into account the husband's order to boycott them due to financial disagreements?

What these two men are doing, denying the Hereafter, is clear disbelief that expels them from the faith if they intended it. Therefore, they must be advised and reminded of Allah. As for the questioner, she must sever ties with them, especially since her husband has already done so and instructed her to do the same. This abandonment is not considered a severing of kinship ties if it is done in a Shariah-compliant manner.