Although unethical behaviors are prevalent in the workplace, little is known about the unethical behavior aiming at benefiting one’s family (unethical pro-family behavior, UPFB). In this paper, we leverage self-determination theory to explore the association between work-to-family conflict and UPFB. Specifically, we hypothesize and corroborate a positive relationship between work-to-family conflict and UPFB, and such relationship is mediated by family motivation. Moreover, we identify two conditional factors, guilt proneness (at first stage) and ethical leadership (at second stage), in moderating the proposed relationship. In study 1 (scenario-based experiment, N = 118), we tested the causality of work-to-family conflict and intention to perform UPFB. In study 2 (field study, N = 255), we tested our hypotheses by employing a three-wave time-lagged survey design. As expected, results from two studies fully supported our predictions. Overall, we explain whether, how, and when work-to-family conflict will lead to UPFB. Implications of theory and practice are then discussed.