1. Since your father purchased the property by a registered sale deed, it becomes his own property, besides he has got it mutated to his name after purchase. Moreover the seller has inherited this property as his share out of his father's share of property, hence the purchase is genuine you may have to strongly challenge and contest the same properly.
2. If there is no restriction to sell by any orders of court, he may very well proceed with his proposal to sell his property.
3. They cannot object, if at all they have any objection they may have to approach court with an application for injunction restraining your father from alienating the property.
4. It is with him under his possession, then why this question, you may have to fight the case filed against you properly to protect your interests.