Can a son (or daughter) challenge parent's will on self-acquired property on grounds of inequality?

I have seen several discussions on written will on self-acquired assets and the ground rule is: "If the asset is self acquired by the parent and a will is made on that property, then it cannot be challenged. Son or daughter - doesn't matter." This question assumes few things: 1. The son (or daughter) cannot prove that the parent was forced to write the will in such a way that the other son (or daughter) does not get an equal share. 2. The son (or daughter) cannot prove that the parent was not sane at the time the will was written. 3. All sons (or daughters) in this case are married with kids and financially stable. What this question is about: 1. Can the son (or daughter) challenge the will that the parent has always been partial to him (or her) since birth and hence the self-acquired property has been divided inequally? Say for example, a daughter says the son always got better school, better education, more money from father for his business, daughter never received any financial support from father after her marriage, etc and hence the son has a better quality of life than the daughter? Is this a valid ground to challenge the will? 2. Can the son (or daughter) challenge the will saying "I am ok if my parent gave away the property to some orphanage, or threw it away to someone else. But if it is to be divided among the children, then everyone must get an equal share"? The basis for this question is, when the parent writes off the property to a third party, then all the sons (and daughters) are still at equal share (which is Rs 0 to each). But when it is divided among the sons (or daughters), it has to be divided equally? Is there any law regarding parents treating their sons (or daughters) with partiality? If there is any such law, can those laws be applied to challenge the will?