I am of course sorry to hear about your situation.
To answer your question about the statute of limitations, there is no statute of limitations (criminally) for this sort of offense in Virginia. The statutes of limitation can be found at Virgina Code 19.2-8, which can be found at this website: http://126.96.36.199/cgi-bin/legp504.exe?000+cod+19.2-8.
Therefore, as a legal matter, there is no impediment to the prosecution. You and the other victims certainly would be taken seriously. If the offender denied the conduct, then the case would ultimately boil down to credibility. Presumably no physical evidence exists, but that is not necessarily a big problem. If the prosecution has several credible witnesses who testify that this occurred (perhaps as corroborated in part by your statements years earlier), then he certainly could be convicted. You would probably need to speak with your local law enforcement and the prosecutor to determine whether they want to press charges.
I will also note that Virginia has many statutes addressing various forms of sexual offenses. One of the primary rape statutes states, "A. If any person has sexual intercourse with a complaining witness, whether or not his or her spouse, or causes a complaining witness, whether or not his or her spouse, to engage in sexual intercourse with any other person and such act is accomplished (i) against the complaining witness's will, by force, threat or intimidation of or against the complaining witness or another person; or (ii) through the use of the complaining witness's mental incapacity or physical helplessness; or (iii) with a child under age 13 as the victim, he or she shall be guilty of rape. VA Code 18.2-61." Va Code 19.2-8. (http://188.8.131.52/cgi-bin/legp504.exe?000+cod+19.2-8). I have not traced the history of the statutes back twenty-four years. The offender would be subject to those older laws. I am proceeding under the impression that they were largely similar to the current laws. (Of course, the laws for sexual battery have continued to evolve, and so certainly there have been several revisions in these statutes.)
This same statute states, "There shall be a rebuttable presumption that a juvenile over the age of 10 but less than 12, does not possess the physical capacity to commit a violation of this section."
Another statute, entitled "18.2-63. Carnal knowledge of child between thirteen and fifteen years of age," states: "A. If any person carnally knows, without the use of force, a child thirteen years of age or older but under fifteen years of age, such person shall be guilty of a Class 4 felony." This Statute can be found here: http://184.108.40.206/cgi-bin/legp504.exe?000+cod+18.2-63. And by clicking "next" and "previous," you can view the adjacent statutes, which deal with similar offenses.
This website has a useful chart addressing the VA statutes regarding rape: http://www.vahealth.org/civp/sexualviolence/varapelaws/statutes.pdf
I will also note that Virginia has addressed civil claims for sexual abuse of minors. Virginia has enacted a statute to address civil claims for sexual abuse, which can be found here: http://220.127.116.11/cgi-bin/legp504.exe?000+cod+8.01-249. This would probably not apply to your case due to the age of your case (occurring prior to this law).
As for a claim against the offender's mother, that is a somewhat novel theory. The criminal law generally reocgnizes that people other than the actor committing the crime can be guilty if, for example, they "aid and abet" the primary offender. In this case, it does not appear that the mother actively aided these crimes. The prosecutor could, however, make a creative argument that she did aid the crimes, by providing the opportunity etc., but this is a novel claim that may be difficult to maintain.
There may be other statutes that would apply, but these also might be a strain to apply to this situation. For example, Virgina Code 18.2-371 (entitled " Causing or encouraging acts rendering children delinquent, abused, etc.; penalty; abandoned infant"), states: "Any person 18 years of age or older, including the parent of any child, who (i) willfully contributes to, encourages, or causes any act, omission, or condition which renders a child delinquent, in need of services, in need of supervision, or abused or neglected" is guilty of a crime. In your case, it seems that a solid argument could be made that she "willfully" caused the condition which rendered the child "abused" and "neglected," if she knew or had reason to know of this conduct.
Finally, although the claim may be time-barred, a person abused may have a claim against a person who allowed the abuse to occur. In one case, the victim sued the psychiatric facility which allowed the offense to occur. The courts held that the victim's claim against the facility could go forward. Delk v. Columbia/HCA Healthcare Corp.
259 Va. 125, 523 S.E.2d 826 (2000).
You may want to arrange a meeting at your local prosecutor's office to discuss this further.
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