Is a brother considered disobedient to his father for marrying without his permission, and is a father entitled to prevent his children from communicating with their brother and their mother under the pretext of filial piety?

There is no obedience to a father if he commands the cutting off of kinship ties, because obedience is due only in what is good and lawful, and cutting off kinship ties is not among what is good and lawful. As for your brother's marriage without his father's consent, his action is not considered disobedience as long as he aims to protect himself [from sin], especially since the father's refusal is not due to the woman herself, but rather to the son marrying before his sisters. It is necessary to try and appease the father and involve some respected figures, without the brother having to divorce his wife to satisfy him, as he is not obligated to do so.