1. If I get a contract prepared where they acknowledge of my share in the property? That neither one of them will stake claim for my share in the property in any marital dispute.
A) Yes you can get a contract prepared but it is again creates disputes when your brother later refused to give your half share and knocks the door of the court. It is better to get a registered gift deed done in your favour (your half share).
2. I will have primary right to request to sell this property at any time and they will have to repay me one half of the share.
A) Follow answer of No.1
3. Primary question is would such a document have any legal standing in court of law?
A) It has legal standing but long disputes will go on in the courts, waste of time, waste of money and mental tension. It is better to get regd gift deed as suggested above.
4. Is there a way to get it registered so in future no one claims that this is a false document and she never signed it?
A) Regd. Gift deed is better.
5. Third, if I ask them to pay a reasonable market rent for my share in the flat, would it help in proving that this is just a rental home and she not claim ownership in that flat?
A) By merely taking rent from your brother will not establish that you are the legal owner. There should be proper documentation (i.e. regd. Gift deed).