I have family and friends who have visited me over these many years. I have always just sent a letter of invitation. However, each one of them said that they were never asked to show the letters.
So, Triple citizen may be right. However, it just makes one feel better prepared and confident in the eventuality that they do ask.
However, tourism,disneyland in particular is a very good reason to visit.
Tax returns,investments, property and earning statements have been asked for.Even if they have not been visually verified, the confidence with which you can say that I can show you my portfolio makes a lot of difference.
I don't know if your sister is in India. But these days its not uncommon for the officers at the consulates to interview successful professionals who have no desire to leave their home country to settle anywhere else. They actually believe you when you tell them that its just a pleasure trip.
Best of luck to your sister.