Assuming that one of the beneficiaries under the will has already applied for a probate order for the will to be enforced at the Inheritance Registrar, once objection is filed there, the case will automatically be transferred to the family court, where full legal proceedings will take place.
Within these legal proceedings the family court will decide whether the will is valid or not i.e. whether it fulfils the substantive and procedural provisions of the inheritance law, and whether your father, the testator, had the necessary legal capacity to make a will at the time, and was free from any undue influence, etc, and whether it represented his last wishes. It wiil consider evidence, both written and oral, including testimony of witnesses.
If the family court upholds the will, despite the objections of one of your siblings, your late father's estate will be divided according to its instructions. If not, it will be divided according to the Inheritance Law of 1965. The court can uphold part of the will,rather than the whole document, so that the estate can be divided up partly according to will, and partly according to law.