Introduction
Relational sin is sin that harms a relationship between two people. Neither person has to be Christian; the principles here come from the Bible, but also from common sense of those who have observed how humans think and work, both intellectually and emotionally.
The one-day rule
Ephesians 4:26 says to not let the sun go down on one's anger. I believe this wholeheartedly, having dealt with anger that stewed over many sunsets. Sun-set anger is tremendous ammunition for Satan, ammunition he has used to great effect, to assault my well-being.
Matthew 5:23-24
if your brother has something against you
23 Then if you offer your gift on the altar, and remember there that your brother has something against you, 24 leave your gift there before the altar, and go. First, be reconciled to your brother, and then coming, offer your gift.
Matthew 18:15-22
if you have something against your brother
15 But if your brother may sin against you, go and reprove him between you and him alone. If he hears you, you have gained your brother. 16 But if he does not hear, take one or two more with you, "so that on the mouth of two" or "three witnesses every word may stand" *. 17 But if he fails to hear them, tell it to the assembly. And if he also fails to hear the assembly, let him be to you as the nations and the tax collector. 18 Truly I say to you, Whatever you bind on the earth will be, having been bound in Heaven. And whatever you loose on the earth will be, having been loosed in Heaven. 19 Again I say to you, If two of you agree on earth as to anything, whatever they shall ask, it shall be to them from My Father in Heaven. 20 For where two or three are gathered together in My name, there I am in their midst. 21 Then coming up to Him, Peter said, Lord, how often shall my brother sin against me, and I forgive him? Until seven times? 22 Jesus said to him, I do not say to you, Until seven times, but, Until seventy times seven.
Ephesians 4:25-27
do it immediately
25 Therefore, putting off the false, "speak truth each with his neighbor," * because we are members of one another. 26 "Be angry but do not sin;" * do not let the sun go down on your wrath, 27 nor give place to the Devil.