How to recover loan given to relative?

- My mom gave a loan of Rs 4.20 lakhs to her sister for her daughter's education purpose on the year 2016. - She did not make loan agreement but had transferred the amount via NEFT. - 1st proof: The bank statement. - On the beginning of this year 2019, my mom asked for repayment details and gave them a loan agreement to sign based on a lawyer's advice, but her sister's husband and her sister hurled abuses at my mom and told that they were not going to repay the amount nor sign the agreement. - 2nd proof: Luckily I had recorded the telephonic conversations between them and my mom and me. It contains details about loan details, abuses, little of family dramas. But the important thing is in that they have acknowledged the said amount. - One important thing they said in telephonic conversation is that only 1.5 lakhs was spent for their daughter's education and the rest was for "other purpose". - 3rd proof: we have a photocopy of original loan agreement sent too. - Now how should I proceed? - Should I complain to police or go to lawyer?