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Children are focused and live in the present moment. Explaining separation in a language that they can understand, being honest and using examples that they can associate with is helpful.
You would want to stress out to her that moms and dads share different kind of friendship than they do with their own kids (to reassure her that you won't fall out of love with her) Then you could explain to her that sometimes grown ups even when they are mommy and daddy to someone can find another grown up that they would want to be friends with. Stress out to her again that the child is always going to be their child but that they can always if they chose to live with another grown up. If she asks why, you don't necessarily have to say they had fallen out of love (sometimes you won't know exactly why and that is the truth/since many factors lead to that final stage)
You can say that you're not sure and that some reasons are that people at times just decide to make new friends or marry someone else. If she asks you if you and her dad can do that you again explain to her that it is a choice the two grown ups make and it does not happen to everyone.
You can still use the examples you had found online and just make up a child story (for example how two burdies lived together in a nest and took care of the eggs and baby chicks and one day decided to fly to another place) and substitute the information so it applies to her auntie.