Well one answer is that we wouldn’t, that’s why most religions are based on faith.
Another answer is that people who have had “near death experiences”, or who have been “clinically dead” (their heart and breathing has stopped) and then revived, sometimes talk about bright lights, image and even experiences.
Another answer that your sister has heard about is remembering previous lives, suggesting a previous life on earth (and therefore “reincarnation”).
Personally I don’t believe in any of that though. There is not really any hard evidence for any of it and there are a lot of scientific theories to explain near death experiences. As for the boy that remembers a past life I’m not sure. I suspect that his story probably wouldn’t hold up to a thorough scientific investigation though!
What your sister is talking about is “reincarnation” where somebody believes they have had a life before the one they are living now. I don’t think any science has been able to prove this, but who knows what we’ll find out in the future.
A lot of people who believe in life after death, or life before this one, have some kind of faith or religion. It’s an interesting idea and I’d love for science to be used to test whether or not it’s possible.
Personally, I haven’t seen enough evidence to convince me – but I’ll always keep an open mind about these things!