Either you can claim income tax rebate of 100% interest & principal repayment by assuming that you have invested in the flat in the name of the third person. The flat will be in your books of accounts.

or

can claim income tax rebate of 50% of interest & principal repayment by assuming that you and your mother have invested 50 - 50  in the flat. The 50% flat will be in your books of accounts.

Since the source is your money and the EMI payment is also being made by you,  you can claim it in your return., assuming your mother did not contribute anything to the cost.  Since the Benami Transaction Prohibition Act prohibits benami transactions other than those involving wife and unmarried daughter, you can safely take credit for the payment.