Yes, anyone who is a US citizen or a US green card holder can act as a sponsor for affidavit of support purposes.  However, the income that person is deriving or a combination of income and assets must be sufficient to act as a sponsor.  It sounds like this would be for a household of 6 (your sister's family plus your husband who is the beneficiary) and the federal poverty guidelines indicate an annual income of $42175 for 2018 to qualify as a sponsor so you might be ok.  please feel free to contact me directly for any additional assistance.