advice after istikhara

brothers and sisters.
well, this thread is just to seek advice after i have made istikahra regarding a subject. im not actually sure if it needs advice form muslims in general, or a scholar. it's not about marriage, so don't get too excited. lol
but anyways, here goes.
A few weeks ago, my sister implied that someone was a kaafir.
now, i know the ruling of these comments very well, and what i have made istikhara about is whether i shoukd tell her about the ruling, which would be telling her that she is a kaafir. What i am confused about and need your advice in, is that she dosn't know about these rulings. i.e what the nullifiers of imaan are. as far as i know, that whether someone knows or not about the nullifiers of iman, and he/she commits such actions, he/she is still considered as a kaafir. what i also know is that this ruling does not apply to someone who lives in a area where knowledge is not widespread.

So what i am confused about is that i don't know whether she falls under someone who has been exused.

let me give you our background. we live in australia. knowledge is widespread (alhamduliah), however, were we live, the only islamic school here opened 2 years ago, and it only goes on to yr 6. and she is alot older than that. we were never taught by anyone about aqeedah matters. we have riyaad us saliheen (which is where i found the hadith), but i don't think she knows the hadith exists.

anyway, like i said, maybe this needs a scholar, but im just seeking your advice after making istikhara.
please don't make any rulings if you do not know.

jazakallahu kahir.