Can my sister be a joint sponsor on my affidavit of support for my daughter?

You fill one affidavit. Your sister also fills one affidavit. There should be no problem.

If your sister is a permanent resident or US citizen, then she can be a joint sponsor if she has the required income. You still have to submit the I-864 for yourself, indicating that you have no income. That should not be an issue.

You must fill and file Form I-864 even if you do not have any income. If there is a co-sponsor with enough income, your having no income will not cause any problems with the case. You have, however, to ask your sisters whether they are ready to support your daughter while you will be looking for work - because, by law, she will not be eligible for any public support. No Medicaid, no foodstamps, no welfare. This is what the Affidavit of Support means: the sponsor promises the U.;S. Government that the immigrant (in this case, your daughter) will not become a public burden, that, if she will need financial support, the co-sponsor will provide it. In the past, the government rarely enforced sponsors' obligations; immigrants applied for public support and received it without serious consequences. Now, the government increasingly enforces sponsor's obligations, in some cases, by denying public benefits to the sponsored immigrants, and in other cases, by billing the sponsors for the value of public support received by the sponsored immigrants. So make sure that your sisters understand what is the responsibility you are asking them to assume.

You as the petitioner must file the affidavit, but your sister can file another one as a joint sponsor.

Your sister, your friend your neighbor or any other person can join you as a co-sponsor if and when you need to submit the Form I-864, and the fact that you do not presently have a job will not be a problem. The person acting as a co-sponsor must be a U.S. citizen or green card holder. The person must be legally residing in any state in the U.S. and it is not necessary for such person to be living in your house or even in the same state with you. You stilt need to fill the I-864 and include all your relevant information which will clearly show that you do not meet the requirements and the co-sponsor will fill another I-864 with his/her own financial documents that will be attached to your own affidavit. It is also possible that your financial situation may improve before you have to fill the I-864 in which case you may not need a co-sponsor.

Yes your sister can be a co-sponsor, however you need to file the I864 as well.