My mother sister and I took voluntary departure in 2008.when can i be able to go back

I am assuming since you came into the US at the age or 4 or 5 that you had accumulated significant period of unlawful presence. Although you do not have a prior order of removal because you took voluntary departure, your unlawful presence has not been waived. Upon departing the US in 2008, you were subject to the 10 year bar for re-entry for unlawful presence. Provided you have not re-entered illegally at any other point, your citizen step-father should be able to petition for you now since 10 years has past.