Query regarding share of property rights

I am a married Christian woman. My mother is retired who has decided to sell her flat and buy a new flat along with my elder sister (also married). Each of them will be paying 50% of the cost. My mother would be giving the money she received from her old flat whereas my sister would be taking a loan. Now we have to make a affidavit stating that after the death of my mother her share i.e. 50% would be divided between my sister and myself. However, my sister says that the interest she is going to pay in EMIs also should be considered. If so the my mother share would stand at 43% instead of 50%. Is it legally correct for her to consider the extra interest she would be paying over the years?