Unmarried sister in India she is 21 only

[binitp18]

Hi,

I will be citizen in this December. And I am going to apply for my parents lives in India for family base green card.
I have younger sister she is 21 years old and not married ever.
she is depending on my parents until she get married.
she do not want to get married before 24 at least.
So my parents can not leave her in India alone. So they can not come.

Question:
Is there any law or special request kind of form or application so that way they all can come at the same time. (which takes usually 1 or so)

what if she comes here on visiting visa and than I apply for her on family based green card?

if not what is the best option?

[immihelp]

She can't really come to the US along with your parents.

Even if she comes to the US on visitors visa, and if you apply for her greencard, it takes 13 years and until then, she can't stay in the US on visitors visa.