If the property was solely owned by Zaid’s deceased Christian wife and the name of Zaid was entered only as formality then it is not lawful for Zaid to make any kind of will for this property. Now the property became the inheritance of the legal heirs of Zaid’s Christian wife. But if the property is owned by Zaid then he has the right to give it to anyone whom he wishes in his lifetime. He may make a will of one third of his property for a person who is not heir (also to an organization provided it is run by Muslims), while the will made towards heirs such as daughter, step brothers etc shall not be valid and executable as per the Shariah. He also can make a will for the children of his real sisters. But the overall will whether it is for organization or for bhanja only it shall be valid and executable in only one third of his property. The rest two third shall be divided among his legal heirs such as daughter and step brother etc. The daughter shall get ½ and the step brother shall get ½ whatsoever is left after the will. Yes, if he gives as gift to his daughter in her complete possession in his lifetime then he may give her more than the said share but it is not good to do so in order to deprive his heirs like step brother etc.
Allah knows Best!
Darul Uloom Deoband