1. your husband can transfer his 50% share in the flat to you by signing a gift deed in your favour
2. since the flat is mortgaged, you will have to take NOC from lender
3. you can become a co-borrower alongwith your husband
4. the GPA given to your husband's dad can simply be revoked by writing a letter to him
5. you can also issue a public notice in newspapers to inform the public at large that you have cancelled or revoked the GPA given to your FIL by so and so letter
6. the gift deed can be signed by you and your husband before the public notary there or before Indian Consulate - latter is more preferable
7. you and your husband also need to grant a power of attorney to some trusted person in India who will confirm your signatures on the gift deed before the registration officer in India and complete all the registration formalities on your and your husband's behalf
8. the above power of attorney has to be compulsorily signed before and attested by the Indian consulate or embassy
9. also inquire with the consulate or embassy if they will require the power of attorney to be apostilled before they can attest the same
10. this will transfer the 50% share of your husband to you in your lifetimes itself
11. your husband can also make a Will in your favour therein bequeathing his 50% share in the house in your name
12. the Will has to be signed in presence of two witnesses
13. but transfer of the 50% share in your name will happen only after the demise of your husband
14. there are also chances that his Will may be challenged by his other legal heirs on the ground that it is forged or fabricated