As-Salam alaykoumI know somebody related to me who doesn't tell his older children they have brothers and sisters in different countries. Mind you the oldest son is 25 years old and he doesn't know his siblings nor the wives of his father. His father never explained why.Is this permissible in Islam?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Undoubtedly, the fact that the man you referred to did not tell his children that they have siblings leads to the severing of ties of kinship and deprives such relatives from their right of keeping ties with them, while Allah Says (what means): {So give the relative his right.} [Quran 30:38]

As-Si’di said when interpreting this verse: "{So give the relative his right.}, of due and recommended kindness and benevolence; this right varies according to the different of circumstances of the relatives, their closeness as kinfolks, their need, and it also varies depending on the time people live in." [End of quote]

Additionally, not informing them may lead to the loss of some rights of inheritance in the future and so on.

Therefore, this man must fear Allah and take the initiative to inform them so that they can fulfill the commandment of Allah of keeping ties of kinship, and so that he does not cause disputes and dissension between his children if he dies while his children do not know each other and they do not know that they exist at all.

Allah knows best.