Letter of declaration

No ,it is not necessary whether other sons are harrassing you for pension or blaming you that you are misusing ,you record a video and keep it for proof as electronic evidence.

Your mother may give any part of pension to you or any other member of your family, there is no dispute.

No legal issue is involved in the facts.

Howeveer, if you are dissatified and have some reservation(s) it is better to consult and engage a local prudent lawyer for appreciaiton of facts/ documents, professional advise and necessary proceeding.

It is only your mother who can raise any such issue as the money belongs to her, and non-else has any right or claim;;;; if she is satisfied with your spending then that is ok. 

But If there are bank transactions or transfers from your mother’s account to your account then your   brothers may in future or after your mother harass you for such things,.So it is better to get an affidavit from your mother stating all the money given by her to you date wise  has been spend on her alone and  no one has any right to seek any kind of accounts from you. You need to get that witnessed. But if the money transactions are in cash you have no accountability. Or get statement of accounts made and signed by your mother. 

Thank you all for your replies.