How can my sister help me apply for American immigration?

She can't apply for you unless she is a relative immigrant. Such as skilled immigrants and investment immigrants. You have to work hard by yourself.

At that time, she herself "had not applied for a green card", that is, she did not have legal residence status in the United States and could not apply for immigration to your relatives.

So your sister can't help you apply for immigration.

Yes, your sister can only apply for a green card. She can help you apply, but it will take a long time.

As for skilled immigrants, I suggest you don't consider it.