You will need the I-864 once your wife arrives on her K-1 visa, you marry within 90 days and you are applying to adjust her status to a permanent resident.
Your stepfather could be a co-sponsor or a joint sponsor. This is a link to an article explaining the difference: http://immigrationlawlasvegas.blogspot.com/2013/08/affidavits-of-support.html.
Affidavits of Support can be complicated and mistakes can have serious consequences. I suggest that you contact an immigration attorney.
I'm a bit confused by your question in light of your fact pattern in that it seems your fiancee is not currently in the USA (the language you use seems to indicate she is outside of the USA right now).
If that is true (that your fiancee is not currently in the USA) then that opens up another can of worms that is "prior" to any I-864 being submitted. That issue is: How is your fiancee going to immigrate to the USA (gain entry)?
If her plan is to enter the USA, get married and file for a greencard, then that means she has "immigrant intent". If she uses a non-immigrant option (like a visitor visa or the Visa Waiver Program) to "attempt" to "immigrate" to the USA, she runs the risk of being turned around at the port of entry (when attempting to enter) for not having the proper visa to match her intent (if she wants to come here, get married and file for a greencard, they you should file an I-129f petition for an alien fiancee... Congress created the K-1 visa for that very purpose). If she's extra "unlucky", not only will she be turned away at the port of entry, she'll be turned away with a finding of misrepresentation that may result in her being banned from the USA for a period of time.
Simply not worth the risk (just my own opinion) when contemplating starting a life in the USA with your loved one. Why expose her to that risk and jeopardize her future in the USA? To avoid the time it takes to get the correct visa (K-1 visa)?'
Know that right now, K-1 visas are processing extremely quickly. My last half dozen I-129f petitions were approved at the CSC in 4 weeks or less and the new DS-160 procedure at the U.S. Consulates seems to speed up that part of the process as well.
Once she enters the USA, marries you, and files for permanent resident status, "then" your I-864 question will be relevant.
Best of luck.
Matthew Udall - Attorney