Is it a particular kind of reincarnation (Buddhist, Hindu, Pythagorean etc.) or is it just First world pick and choose "faith"?
Because if it's the former, there are certainly "definite" (e.g. Samsara and Nirvana). Is she trying to escape from Samsara? Because if not then her stay here will be very much definite.
Buddhism is morally redistributive (i.e. good acts can be rewarded, bad ones can be punished etc.), but there is no real reason to. Time in such a calculation is basically infinite, so you don't need to be in any rush to better yourself at all, and therefore it is a cop out.
For us, we have personal allegiance to Christ and His promise of redemption and salvation, and we have to have charity to others, and we as Christian have to think of salvation in plural, we are certainly our brothers' keepers, and we do well to help our brethren along the straight path. Buddhism has none of this, not even to Buddha himself. Essentially it is a selfish, regressive nihilism that precludes the need to try to live a life of any worth or point. No virtue or love or warmth is necessary, and you an just close your eyes to evil, because evil itself is just goodness in need refinement (apparently). There is no sense of duty or loyalty, because at the end of it all you are just going through the motions to get Moksa. Buddhist societies are also deeply uncharitable and proud, especially places like Tibet, where beggars are viewed as irredeemable scum who are being punished for the sins of previous lives, and are therefore ironically trapped to keep repeating their lives, since they are not allowed any social/spiritual mobility. This also ties into our idea of suffering as penance and as purgation, things to be embraced to imitate Christ. For them however, suffering is to be avoided at all costs, since it is seen as evidence of your wickedness in a past life. Oh, and it also believes the material world is essentially an evil prison, rather than the good work of God's creation, which is heresy.
If on the other hand, she believes in a Celtic/Pythagorean form of reincarnation, then the present life is irrelevant because you will just come back at another time, and nothing that you have done in this life, good or bad, will make a stitch of difference. Sure it might make you brave and outgoing to know that you will come back some time, but when none of your actions are being held to account, what good is it for you?
If it's the latter, just pray for her and be an example to her, because arguing with that kind of "spirituality" is like swimming through treacle.
I will pray for you and your sister.