The I-864 is for your sister and she has three derivatives with her (husband and two minors). You will ahve to fine an affidavit no matter your income as you are sponsoring them. You need to file as many co-sponsors as will meet the threshold as set by the poverty guidelines. Each joint sponsor needs to have a full affidavit done as if they are the only one with an affidavit.
The DOS has an excellent set of Q and A's for related issues
USCIS also has good general information
You can also contact USCIS customer service and speak with an agent; state you have questions on the Affidavit of Support. They have very good manuals they use to guide you through the process.
Legal disclaimer: The statement above is provided by CC Abbott is based on general assistance and not intended to be a legal opinion because not all the facts are provided. The person requesting information and all others reading the answer should retain an attorney who is permitted by the state bar within the jurisdiction who can examine the complete facts and provide a legal opinion on your case. All information provided in the above answer and other information provided by CC Abbott does not create an attorney/client relationship within any state of Federal law.
I agree with my colleagues.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship.