All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The matters related to inheritance are serious; it is not adequate to seek a Fatwa in this regard. Rather, you are advised to refer the case to a legitimate court of the Islamic law so that an Islamic judge can investigate the case, evaluate the deceased’s estate, identify the eligible heirs entitled to inherit, and give each his due share in the estate. In the case of a minor heir, he (the judge) would preserve his wealth or appoint a legal guardian of it.
It should be noted that one of the most important objectives in the religion is to promote amicability and affection among Muslims. Allaah, The Exalted, says (what means): {The believers are but brothers} [Quran 49:10] Ibn ‘Umar reported that the Prophet, sallallaahu ‘alayhi wa sallam, said: “The Muslim is the brother of his fellow Muslim.” [Al-Bukhari and Muslim]
The obligation of maintaining this fraternal bond is more emphasized in the case of those who are related by blood or marriage relations.
If the case is as you have described in the question and this woman is hardening her child’s heart against his paternal grandparents and uncles and preventing him from maintaining his relationship with his father’s family, then she has committed a grave sin and has caused severance of the ties of kinship which is a grave sin in Islam, that incurs the wrath of Allaah and the deprivation of His mercy. Allaah, The Exalted, says (what means): {So, would you perhaps, if you turned away, cause corruption on earth and sever your [ties of] kinship?} [Quran 74:22]
She has also committed a sin by imitating what magicians and sorceresses do in sowing discord among those who should hold affection for each other and by severing her relationship with her husband’s parents, if she did so with no valid reason. It is impermissible for a Muslim to forsake his fellow Muslim without a valid reason, as established by the authentic traditions of the Prophet, sallallaahu ‘alayhi wa sallam.
Our advice is to try to achieve reconciliation and settle the disputes; Allaah, The Exalted, says (what means): {No good is there in much of their private conversation, except for those who enjoin charity or that which is right or conciliation between people. And whoever does that seeking means to the approval of Allaah - then We are going to give him a great reward.} [Quran 4:114]
As for that woman's marriage to the brother of her dead husband, if it took place after the end of her ‘Iddah (waiting period), with the consent of her Wali (legal guardian), and in the presence of two witnesses, then this is a valid marriage according to the Sharee‘ah. The fact that it has not been announced is of no consequence since announcing the marriage is not obligatory according to the preferred opinion of Muslim jurists. It is impermissible for this woman to request divorce merely because her husband’s family learned about their marriage.
On the other hand, if you mean that she and her dead husband’s brother got married secretly (without the knowledge and consent of her guardian and the presence of witnesses), then this is not a marriage; it is plain Zina (fornication/adultery) and it is impermissible in all cases.
Allaah Knows best.