Delay in registration of the property

Hello Sir/Madam, My mother purchased ground floor of a 2 stories building from her own brother. This property was owned by my maternal grandmother who gave as gift deed to her grandchildrens of her sons only. We purchased the ground floor in 2005 in cash as they wanted. We were told that registration will be done within 6 months however it didn't happened as one of the daughter was in USA and was told that once she will be back they will do registration. But she didn't came back until one unfortunate incident happened when younger daughter died in 2007. That time due to situation we couldn't spoke properly about registration. Next year my Father passed away. And then they started avoiding the registration. In 2010 they sold their upper floor to other people. Recently while going to USA they asked me to get our documents ready as they will do the registration once they will be back in December so I checked the registration cost with department it is more than 2 lakhs which was 36000 in 2006. We are living in this house in very bad condition. I request you kindly let me know who will be responsible to pay so huge amount. What are the option so that we need to pay less. Looking forward for your kind help.