Yes. The fiance needs his own I864 as well.
The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter,not should it be viewed as establishing an attorney client relationship of any kind.
You will only need to file for an I-864 for your sister if you are her original petitioner AND if her priority date is now current and she is able to file for an immigrant visa at a US consulate abroad or an I-485 if currently in the US. Given that you do not seem to meet the i-864's financial requirements you will need to find a co-sponsor. Your fiancé, while not a "household member" by your own description, can nevertheless serve as the co-sponsor, provided he meets the financial requirements.
Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.
If you are the petitioner you must complete the I 864 with all required documentation even if you are unemployed. The co sponsor or household member will need to complete the proper forms and file the appropriate documents as well.Read the instructions for each form carefully or retain an experienced immigration attorney to assist you.