The gold is a moveable asset. So it may be taken away by the aunt without any reference to anybody
But I do not understand how she was operating the bank account?
Was she operating the account which was in the name of your father or in joint names of your parents?
Was the aunt appointed as a nominee of that account?
Did your father transfer half his property under a registered transfer deed in favour of your aunt?
If the aunt or his son were made only half owner then how could they rent the entire house?
I assume it is an ownership flat in society
The facts given by you in your query are incomplete and vague
In order to reclaim the property and other assets you will have to file a suit against the son of your aunt
If it's your claim that the flat is yours after your parents demise and that the rent and deposit are not paid to you, this too needs to be claimed in a civil suit as above
And what kind of documents are mentioned in your father's letter
Your entire query is bereft of essential facts and just too vague
You need to elaborate so that a proper response or atleast something near thereto as possible can be given to enable you to decide the next course of action
Presently my reply will be a limited one given the incomplete facts provided