Can I file a civil suit against my stepbrother for molesting and raping me when I was a child?

It depends on the statute of limitations for this type of claim in the state in which it took place.  The following is from a website id Greene Broillet & Wheeler LLP discussing California law, but other states may be different. Childhood sexual abuse survivors must file a lawsuit by age 40 or within five years of discovering the abuse as an adult. As of January 2020, childhood sexual abuse survivors now have a three-year window to file sexual abuse claims that have previously passed the statute of limitations.   It also may depend on when your stepbrother turned 18, as the limitations period, and the nature of the claims may be different depending on whether he was a minor or an adult at the time of the assault (from what you've written, it seems as if he  was a minor for some of this and an adult for some).  Also, if he was a child at the time of an assault, you may have a claim against his parents/guardians.