Firslty, as per the information mentioned in the above query, makes it clear that you are children from the second marriage.
Secondly, a daughter is already there from the first marriage.
Thirdly, though it may not be necessary for you to obtain the NOC from your sister, and also if you wish then you may don’t have to reveal the same before the authority.
Fourhtly, problem would only come if you have a doubt that she may come and ask for her share.
Fifthly, otherwise if she doesn’t come then there may be no problem even if you don’t reveal of her truth being your sister from the first marriage of your father.