You need not buy from them, you need not co-own and share apartment with them, you need not take NOC from MIL if you do the way I said. Without spending a single paisa, you can get apartment in your name and delink them totally from that property.
What happens to money already paid to bank?
Say you have raise a loan of Rs.8.00 lacs. As on today Rs.7.50 lacs is outstanding and you have paid Rs.2.50 lacs towards loan). If you avoid payment of instalments bank will declare your account as NPA with in 3 months. And tell them to serve SARFAESI notices and take possession of that apartment. They have several options to sell the property and appropriate towards loan amount. One is open auction, other is private treaty, sealed tenders etc. For entering into private treaty with some third party, they need your consent. For open auction and sale through sealed tenders they don't need your consent. Bank prefers private treaty because it is easy process they don't have to conduct auction give sale notice paper advertisements etc.
Some banks tell the borrowers directly, "if we sell your property through auction, you may not get the desired price for your property, so you get the buyer yourself and get it sold by marketing your own property for best price".
So in that process you will take the buyer to bank and tell the bank this person is ready to buy the property for Rs.10 lacs. Bank's outstanding dues is Rs.7.5 lacs. It will appropriate Rs.7.5 lacs towards loan and give off rest of the amount to the borrower i.e. you. Rs.2.5 lacs you have already paid is credited to loan account it has not gone anywhere. Try to understand. Over and above loan amount of some sale proceeds remain the bank will give it off to the borrower.
Bank gives sale certificate to your son and it becomes his "self acquired property". The sale certificate issued by bank in favor of your son will be free from any sort of encumbrances. Nobody has a right over your son's "self acquired property" including your MIL. That way you will get relieved from your MIL's claims over her son's property. After some period of time, say six months or a year, take the son's property in your name through some gift deed/sale deed.
I am saving you at least Rs.50000/- rupees advocate fees and Rs.2.50 lacs that you wish to pay to your MIL.
The procedure I suggested is absolutely legal.