There is no guarantee for visa issuance. You can act as financial sponsors but the bigger issue may be proving that your sister will depart the US after her temporary stay - she must show that she has substantial ties in her homeland or country of residence.
Did you file all necessary and supporting documents? have you gone through Biometrics yet? Do you have any criminal background? These could be factors in the delay, although with the increased numbers of filings, these cases are taking longer to process than before.