1. Well, it depends on your sister situation.
2. The US immagrantion dept will contact your elder sister in US and ask her to do a sponorship application, then when it is completed and accepted. Your second sister will be notified by the HK side for interview. She has to pass the interview and then she will get the visa approve. Once the visa is approved, there is a expiration date and she has to report to US before the deadline.
3. Most likely she should stay at least for the first year in order to receive the green card (residential card) then she can travel to HK but she can not leave US more than 6 months otherwise US has the right to take back the green card. Until 5 years (if she travel more than 3 months oversea,then she has to wait for 5 yr and 3 month, so on and so forth.) then she can apply for citizenship. Once she becomes a citizen, she can go back to HK and stay as long as she wants.
4. When she arrive US and pass the custom, the staff will talk to her and put a VISA stamp on her passport after review all the documents, the number will be A1234567 blah blah, it is her green card number, later on the green card will mail to her. The address will usually be your elder sister address.
5. The green card offically name is premanent residential card. So she is considered premanent resident once she arrives US.
6. If she gives up the chance, your elder sister has to do the application again, there will be appication fee, lawyer fee ........plus, sister relationship is on average 10 yrs waiting now and US once considers cancel it. Also, your elder sister maybe upset after all the works she has done and your second sister just says no and don't go.
Hope it helps! :-P