Agree with PeterB, of course and will only add this:
Don't try to convince your sister of anything, it might cause contradiction.
Sorry, Zen, not Theravada, but fits so well:
A Cup of Tea
Nan-in, a Japanese master during the Meiji era (1868-1912), received a university professor who came to inquire about Zen.
Nan-in served tea. He poured his visitor's cup full, and then kept on pouring.
The professor watched the overflow until he no longer could restrain himself. "It is overfull. No more will go in!"
"Like this cup," Nan-in said, "you are full of your own opinions and speculations. How can I show you Zen unless you first empty your cup?"
You could feel like caring about your next life, if you think about this:
A human rebirth is said to be extremely rare. We have a big chance now
to realize the Dhamma and make sure our next rebirths are not in lower realms, such as hell and animal kingdom, etc,.
Being an animal is not such a joyride...
A Buddhist wants to exit Samsara.
She pointed out that: between Terasi and Teraso there is no memory link, nor there is any knowledge of each other, so those two are basically two different people.
You are not 'bereft' of what you learned however. What you learned will bring you into your new life.
If you learned, understood and lived compassion, it will have a beneficial kammic impact on your next rebirth.
Plus, highly develpped people are said to recollect previous lives.
And the question is: in the selfish point of view of a mere human full of defilement, why should Terasi struggles to cultivate herself just for the benefit of "someone else" (Teraso)?
You're not THAT disconnected from what becomes. You can't inherit anybody elses kamma, only your own.
YOU won't be there anymore, but you will have another awareness, and suffer if you messed up.
You could of course diminish this.
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