How is it possible to forgive and forget?

Anonymous
I'll put my sister as an example. Her (then boyfriend's) now husband's mother and sister were nasty, toxic people to her for a couple years. They all settled their differences by 2015. She's been married for 3 years and they have a daughter.

Since then both have sincerely apologized to her for all their wrongs. She has forgiven and forgotten. My sister is now loved by them and vice-versa. She actually considers them her second family. Similary my sister forgave her former HS bully and even befriended the girl. Till this day, they are good friends.

That's her but honestly how can someone forgive and forget? I can't do that. I can forgive but would never consider them a friend (much less a family), get close to them nor ever trust them. My sister somehow can. How is that possible? Wouldn't they get reminded at some point?
Updates:
14 d
In a way she won in the end. Her former wrongdoers (enemies) now feel like they forever have to make it up to her. Meanwhile she doesn't want them to continue feeling guilty. Still how can a person do that?
How is it possible to forgive and forget?
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