Father can execute will bequeathing shares lying in demat account to you 

2) will should be attested by 2 witnesses . He can also add you as nominee in demat account 

3) sister cannot during her father lifetime waive off her right as she is one of legal heirs only on father demise 

 Instead of your sister executing a Reliquishment Deed to relinquish her right to inherit the demat accounts investments, returns and it's future investment and returns in your favour when your father is alive, let your father execute a WILL and name only you as  the beneficiary of the above investment. In that case, no other person, including your sister can inherit the proceeds of the demat account in your father's name, except you.

If your father makes  A Will, preferably , a registered one giving all his money from investments then your sitter will not acquire any inheritable interest in the said demat account.

The investments made through your father 's demat account will be considered as his own and nobody can claim any share in it as a right during his lifetime.

Hence it is advisable that he transfers the account to your name when he is in full conscious.

Your sister 's disclaimer at this stage will have no effect neither it is legally valid.

Yes you can execute registered relinquishment deed for the same from your sister

Better liquidate the holdings and operate a separate account in your name to avoid litigation.

Regards

G.Rajaganapathy

Dear client,  

As per the Indian Succession Act, all legal heirs have an equal right to inherit the property of the deceased. Therefore, your sister has an equal right to inherit the demat account investments along with you. However, it is possible to create a will stating that the entire demat account will be transferred to you after your father's demise.

Alternatively, if your father is willing, he can gift the demat account investments to you during his lifetime. This would require executing a gift deed and the payment of stamp duty on the transfer.

It is important to note that any such transfer or gift should be done with the help of a legal expert to ensure that it is executed properly and is legally valid.

Your sister has no legal claim over the demat account in your father's name or over the proceeds in the form of dividends and returns during your father's lifetime. However, it is advisable for your father to leave a Will bequeathing all his financial assets covered in the demat account in your favour, in order to preempt any claim from your sister later.

For this your father have to execute in WILL in your favor.