You're asking if you can object to your cousin selling a house he purchased from your aunt, even though it was originally your grandfather's property. While you're willing to buy it at market value, your cousin is refusing to sell it to you. The property shares a survey number with your family home. Unfortunately, once your aunt sold the portion to your cousin, it became his property, and he generally has the right to sell it to whomever he chooses. While you might explore seeking an injunction by claiming it as ancestral property or checking if a 'right to preemption' exists and applies in your specific location, it's unlikely you can force him to sell it to you. Consulting a local lawyer is highly recommended for professional advice on your specific circumstances.