Hi, you can execute release deed in favor of your father and sister and same has to be registered before the Sub-Registrar. 

You can execute a registered relinquishment deed in the office of local Sub-Registrar. You can also register a gift deed to her. Stamp duty for gift deed is minimum. But it is better not to take any decision in haste as property also belongs to future kids, you have no right to deprive them of the same.

If the property is ancestral, you acquired right to share it by birth and you relinquish it or gift. It will be your unascertained share which will take effect when the actual partition takes place in future.

During your father lifetime you have no share in his property 

on father demise intestate only you would have on e third share in property 

you cannot execute any relinquishment deed during father lifetime in respect of his self acquired property 

You cannot execute any relinquishment deed at present in respect of his self acquired property 

1.  Inheritance question comes only if the properties are your father's self acquired and if your father dies intestate ( without executing WILL).

2.    However if it were to be Ancestral property, then you can execute a registered Relinquishment Deed in the jurisdictional Sub Registrar's Office, at the time of partition of the property, in favour of your younger sister.

See, after your father you have relinquish rights if any property is to be inherited.

Otherwise if its your father's self owned property he can make WILL or Gift deed to any other person from your family or outside family.

You don't have any rights in your father's property at least not during his lifetime,  hence there's no necessity or even a question about relinquishing your rights in the property that never is yours. 

You can orally express your unwillingness to acquire any share out of his property not only now but also in future .

You cannot execute a relinquishment deed for the property that was never yours. 

Dear client, A Property can only be relinquished by an Owner of the Property and only in the favor of another Owner, i.e., Any Inherited Property cannot be relinquished to any third person. In other words, only in the name of another Legal Heir can a Property be relinquished.

Any Relinquishment over a Property is made through Relinquishment Deed or Release Deed. Section 17 of Registration Act 1908 makes it mandatory for a Relinquishment Deed to be registered.

Such Deed for the purpose of Registration should be presented in a Proper Format in the Office of Sub- Registrar of Assurances under whose jurisdiction the said property lies in the presence of two witnesses. Any Relinquishment made in favor of Minor attracts the provisions of Indian Contract Act.

Unregistered Relinquishment Deeds are held not to be permissible evidence.

As you have put it rightly, you have not inherited anything as on date, and so the question of relinquishing your claim does not arise at all.

-  You cannot get any share in your fathers property during his life time , and hence you cannot release your share to anyone after executing Relinquishment deed. 

You can relinquish your rights by executing a relinquishment deed.