What is the limitation period for registered sale deed?

My father had two children me(son) and my sister. He purchased an agricultural land in 1968 and it was registered under his name in SRO, he passed away few years later. In 1992 I sold the land to another person, there is an unregistered pre sale aggrement with the buyer which I had signed. The buyer paid me the full amount however he did not register the property with SRO at that time. Patta of the land is still in my name. There is no The buyer has approached me for registering the property now. I am willing to register it to their name without getting any additional money. However I have below questions. 1) What is the clean way of registering the propery to the buyer now? 2) Will there be any tax liabilities for me if the registration is done now? 3) Do I own the property in full? Or does my sister needs to sign? Please advise.