Registration

Greetings! I had booked an apartment in mine and mothers joint name in 2013.The developers have given all thee recipts in my mothers name since hrs was to be the first name.They have not mentioned my name since it was additional headache for them The property they say will be registered in mid jan 2016.My mothers health is failing and i dont know is she will last till that date.1/5th of the amount has been thru her finances which i will payback with interest i soon as i get my invested monies back,The balance of 4/5th money is mine.I would like to know what letter/document i need to take from her so that in case of evntuality the preoperty could be registered in my name.The objections wouldnt be raised by my sisters as they already know that it was purchased in her name and the part amunt that i am to return to her.The objecton may be raised by the builder and i maybe asked to get certain papers /document to prove my claim.Please let me know as soon as possible since my mom's health is serious and cant know what will happen any moment.Thankin You Meena Kini 5