If you really believe that the ECO has made a clear mistake with evaluating your financial standing I would suggest an immediate request to the ECM for reconsideration based upon the mistakes of the ECO.
Appeals can realistically take a long time and are available for Family Visits.
Considering the cost of the Family Visit Visa you may feel that re-application offers the optimum route.
Do be aware that ALL visitor visa's are notoriously tricky to secure.
What did the refusal state?
What duration did you request?
Did your sister sponsor you?
Did your sister supply full bank statements and accomodation details?
