Is one of the siblings taking a portion of the shared parental retirement salary without informing the rest considered theft?

None of the partners is permitted to dispose of common property except with the permission of the others, unless the disposition is for a common benefit. As for money that siblings place in their home, if its purpose is for consumption and household expenses, then there is no blame on whoever takes from it for this purpose, and the reference for this is custom and the consent of all. However, if it was placed for other than that, then it is not permissible to dispose of it except according to the purpose for which it was placed. Whoever takes something from it without the knowledge of their partners is an aggressor and is liable for what they took until they return it, according to the saying of the Prophet, peace and blessings be upon him: "Upon the hand is what it took until it renders it."