ฉันสามารถยื่นฟ้องในคดีแพ่งต่อบุคคลเพื่อเรียกร้องความเสียหายจากการหมิ่นประมาทบุคคลได้หรือไม่ และฉันมีพยานหลักฐานเพียงพอหรือไม่

In most jurisdictions a defamation claim can succeed only if (1) a false statement of fact about you was communicated to at least one third‑party, (2) the statement was “published” (i.e., shown to others in the WhatsApp family group counts as publication), (3) the statement caused or is likely to cause serious harm to your reputation, and (4) the defendant cannot rely on a valid defence such as truth, honest opinion, or privilege; calling you a “narcissist” is usually treated as a subjective opinion rather than an objective factual allegation, so a court may view it as protected speech unless you can show that the accuser meant it as a factual diagnosis (e.g., implying you have a clinical personality disorder) and that the assertion is demonstrably false; your screenshot is useful proof that the remark was made and who saw it, but on its own it may not be enough to overcome the opinion defence, and defamation actions can be costly and time‑consuming, especially in family contexts where courts often encourage mediation or reconciliation; before filing a suit you should obtain a brief opinion from an attorney (who can assess the specific defamation law of your country, the likelihood of success, and potential damages versus the expense) and also consider whether a cease‑and‑desist letter, a mediation request, or, if the messages amount to harassment, a separate claim for abusive communication might achieve a quicker, cheaper resolution.