Forgiveness (as used in the Bible) means to pardon. Not to hold someone’s wrongs against them. In Jeremiah 31:34 God promises Israel in the New Covenant that he would “forgive” and “remember their sins no more.” Sometimes people say: to forgive means to forget. But that is not quite true because no one can really forget. Even when the Lord says he will remember no more, it does not mean he forgot, but that he forgave; meaning he will not bring them up again. Their sins are removed, put away, never to be brought up or held against them forever. Thank the Lord, that is what we have: “forgiveness” which is made possible “through the redemption that is in Christ Jesus” (Ephesians 1:7). Because He paid for our sins, God can put them away (on Him) and pardon us – eternally. I hope that helps.
I was thinking yesterday about your prayer request and statements you made. You and your family can learn to forgive your sister, but she must earn your trust. That will take time and ought to because she has proven that she cannot be trusted. You can give her a clean slate, but your trust she must earn over time.