As email is a type of written communication; and it can give rise to a type of libel that can be pursued through a defamation claim. Since several people live in one state I doubt that you could file a case in federal court under the provisions of "diversity" jurisdiction. Thus you will need to file in state court either in Virginia or Maryland as defamation is a matter of state law, and the cases and technicalities vary from state to state.
Generally speaking (since I am not an attorney in Virginia or Maryland I cannot give you state specific legal advice, but only legal information, if you seeking more specific information you should seek a local practitioner) you will need to prove the elements of the tort of defamation, that words were "published" (i.e. spoken to a third party) and that they were false (truth is a defense to defamation) and that damages where sustained by the plaintiff or plaintiffs (the person or persons bringing the legal claim). This may be time consuming and expensive to take this suit to court and it may cause more family strife, you need to think about this carefully and consider if you might better spend your money on family therapy or some other way to heal the rift in your family.
Since the email is sent via the internet you will also need to prove that it was sent from whom it was sent and that it was authored by the person you are suing for defamation; You may need expert testimony if you cannot otherwise establish the credibility of these emails coming from the person you suspect are sending them as emails can be "spoofed" to appear to be coming from someone when they are not authentic.
Often rather than taking such a claim through court the parties or their lawyers endeavor to obtain an apology or statement that clarifies the situation. Also it is important to point out that people are entitled to their opinions and those opinions may not necessarily be good, but if they are labeled as opinions they may be harder to show to be defamation and even if something is defamatory the parties may only be entitled to "nominal" damages, i.e. the award of one dollar to the party that was defamed. After spending considerable legal fees such an award can be a pyrrhic victory.