What is the ruling on secretly saving a portion of money from the family in the event of a prior agreement not to save?

Helping siblings is among the noble deeds for which a Muslim is rewarded, but it is not obligatory. Saving money without the siblings' knowledge is permissible. An agreement not to save without the knowledge of others is not binding, as it falls under the category of a promise. The majority of scholars (Hanafis, Shafi'is, and Hanbalis) hold that fulfilling a promise is recommended but not obligatory. The Malikis believe that fulfilling it becomes binding if the promise is linked to a reason or if the promised party has incurred an expense based on it.