• Advantages and Limitations of Gift Deed

Dear Sir,

We have a shop in Mumbai which was originally brought by father in the name of my mother.
8 years back she handed over the keys to my sister's husband for him to keep his goods there as he did not have any space of his own to keep them.

NO written document was made/ No Leave and license agreement was made, it was given on basis of pure humanity. However my mother had said she is thinking of giving it to my sister.
All these years light bill and society's maintenance bill is being paid by me as I am looking after my mother.

Now, in the course of 8 years we have developed bitter relations with their family.
When she is asking for keys, they have refused to hand over the keys and instead have put their own locks there.

My mother is very old. She has offered to transfer the shop to me by a "Registered gift deed". Can this be done? Can my sister later file a case/challenge the gift deed?
Would I be able to take possession of shop, by breaking his locks after gift deed is done?

Please suggest and advice on the same. I am very disturbed at the turn of events..