1. Your aunts can sell their undivided share in the property to your father or anyone.
The other co-sharer cannot object to this.
Well it is still not certain that which portion of the property belongs to your aunties hence your father has to file a partition suit against his brother to claim his 1/4th share share as well as the shares of your aunts which he has purchased vide a registered sale deed document.
Your uncle's claim that his permission was not sought to buy/sell this share of property is not maintainable in law.
2. You have to file a partition suit and seek partition and separate possession of your rightful and legitimate share in the property and if necessary you may file a an application before court to appoint an advocate commissioner to inspect the property, divide into such equal shares as per each individual's entitlement and submit his report recommending the allotment of your father's share accordingly.
3. Mutation and partition can be done now, i.e., even after purchase of the undivided share, there is no legal infirmity in it.
4. The police will not interfere in the civil matter until you people indulge in violent activities or quarrels turning into assaults.
5. You may lodge a complaint against them if they are indulging into any excess activity against you people.
Your uncle's son cannot influence the higher police officer, if you approach one seeking his intervention if this guard plays any mischief..
6. Your father can sell his undivided share including the share he purchased to the buyer who be willing the same as it is.