Hi i m nepali passport and green card holder of usa and i want to visit my sister in hk...

You can apply for a tourist entry permit to visit your sister in HK. You should first prepare the required documents containing copy of your passport, the completed application form affixed with a passport-style photo, proof of your financial standing, flight itinerary, accommodation arrangement, and an invitation letter from your sister.
After, ask your sister to submit the application to HK immigration department to apply for it.