Maintenance/custody for differently abled sibling

So you are pressurising the old lady to transfer the said property (flat) in your wifes name or your name for the so called seva that you are doing to your sister in law?  I dont think your mother inlaw would herself have thought of putting some property in her daughters name who is specially abled, as you clearlly mentioned that other sisters are well off.

The other sisters can also have equal share in the said property as per Hindu Succession Act.  So your attempts to gobble up the said property in the name of seva will be futile, chance of you and your wife going to court are quite high, if any of the sisters claim a share in the property.

I advise you as follows,

Your daana dharma and seva to your sister in law will accrue blessings and a seat in his Kingdom of Heaven.  Let it be like this only, dont bother about getting the said flat transferred in your wifes name or your sister in laws name, anyways in India you cant avoid litigation if you are a hindu and have siblings and have a property.  Good luck.