Nothing said or written to you can constitute defamation. Defamation is provably false facts -not opinion- written (libel) or spoken (slander) to 3rd parties that damages the target. If the lies are about you committing a crime, or about you having a disease, or about your chastity (this is an old law), that's "slander per se," which means special damges are presumed and they don't have to be proven.
Lawsuits for these personal injuries have to started 1 year from the defamation. Assuming these events are still within the statute, you should only consider suing your sister if 1) you have a fair amount of money for this expensive litigation, a substantial tolerance for being in litigation for awhile, including an ability to hear and read these lies being repeated over and over again so you can disprove them, and a willingness to drag your reputable witnesses into this dispute 2) your reputation was stellar before this happened and your damages include economic harm to you, and 3) your sibling has money to satisfy a judgment.
No one can advise you about whether this is too big a bother, but you do need to consdier the money, time and emotional toll this would take. If you think this is something to want to do, contact a lawyer.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
Sign up to receive a 3-part series of useful information and advice about personal injury law.