Read 1 Thessalonians 4:1-8

Never harm or cheat a fellow believer in this matter by violating his wife, for the Lord avenges all such sins, as we have solemnly warned you before. 1 Thessalonians 4:6 (NLT)

Sexual sin is particularly destructive of relationships because of the loss of trust it creates. If we remember that the other person who would participate with us in this sin (or be affected by it in the case of the other person’s spouse) is our brother or sister in Christ (or a potential brother or sister, in the case of current unbelievers), the incestuousness of the relationship may help us resist the temptation.