You are not required to do anything as of now
If your siblings claim that they have a right and share in the flat, it's for them to prove the same by filing a suit against you
Presently it appears no suit is filed
So there is no impediment for you to sell the flat
Your late father instead of gifting the money to you, used that money to buy the flat in your name. There's nothing wrong in that. It's not some benami transaction where the true owner is the father and you being his benamidaar
The sisters claim that the flat is the joint family property and pursuant to an amendment in 2005 to the Hindu Succession Act, they being daughters, are entitled to claim a share in the flat
For making such a claim the daughters will have to prove joint family nucleus
They will have to show that the money used for buying this flat belonged to the joint family or was got by selling some joint family property
As regards the deed which you made your brother sign against the money you gave him on the advice of your father , it was in the realm of 'spes successionis' or doctrine of heir apparent or future heir
So assuming the flat was owned by your father, when the brother made the deed, your father was alive. When owner is alive, his future heir or heir apparent cannot have any claim over the owner's property. If he doesn't have any claim then he cannot release or relinquish any right as well. Thus the deed is inconsequential. At the highest it can be said that that was only for your safety. Just as now your sisters are claiming, your brother could also claim. So you just wanted to exclude him from laying such a claim in future by making him sign that deed.