Is it permissible for a sister not to inform her brothers about her son's marriage contract after they have repeatedly failed to invite her to their private occasions?

This worldly life is diversion and play, a realm of trial and test. Among the greatest means of attaining God's pleasure is maintaining kinship ties (silat al-rahim), which is one of God's commands to the believers. Severing kinship ties is forbidden. Abu Hurairah reported that the Messenger of Allah, peace and blessings be upon him, said: "Indeed, Allah created creation until, when He finished, the womb stood up and said: 'This is the station of one who seeks refuge from being severed.' He said: 'Yes. Are you not pleased that I should connect with whoever connects with you, and cut off whoever cuts you off?' It said: 'Yes.' He said: 'Then that is for you.'" Then the Messenger of Allah, peace and blessings be upon him, said: "Recite, if you wish: 'Then would you perhaps, if you assumed authority, cause corruption on earth and sever your [kinship] ties? Those are the ones whom Allah has cursed, so He made them deaf and blinded their vision. Do they not then reflect upon the Qur'an, or are there locks upon [their] hearts?'"

Maintaining kinship ties is not a reward for connection; rather, it means connecting with those who cut you off, doing good to those who harm you, and giving to those who withhold from you. Islamic law has not specified a limit for maintaining kinship ties; rather, its determination is referred to custom. If custom considers not inviting siblings to a son's wedding as severing ties, then she is sinful for that. Otherwise, there is no blame on her as long as she maintains ties with them in other ways. It is preferable that she informs them and responds with that which is better.