how can i get two visas for me and my sister if my friend in china (us citizen) want to invite us?

First, you can ask your friend to sent your an invitiation letter as well as a copy of his passport. The faxed and printed copy are both acceptable. Then, prepare your passports, copies of passports, application forms, qualified bare-headed photos, and other documents you may need, including financial statement, round-trip tickets, etc.. Next, submit all materials to the embassy for approval.