Is it permissible to transfer a sum of money designated for my orphans from a charitable organization to other orphans, knowing that the organization refused to directly assist the other orphans, and would this be considered a deception that incurs a religious burden upon me?

If the giver of charity or their agent stipulates that the charity be spent in a specific place or for a specific entity, their condition must be adhered to. The amount that reaches your account is a right for your eligible children, according to the condition of the giver of charity, and your sister's children have no right to it. It is not permissible for you to give charity from your children's money without the permission of the mature adults among them.