Astaghfirullah!
This is very sad, but not unexpected.
Girls of such age should not be taught by male teachers away from the supervision of other adults.
Islam stresses prevention so that such abuse can't take place.
I would not send my daughters (specially in the ages 12-15) to a male teacher, more so when he is not supervised, in the first place.
Only appearance doesn't make a person pious or just because he is a teacher of religious studies or becuse he comes across as such.
And even pious people can slip and then that slip can become a routine.
In my opinion, this teacher has already gone too far and should be avoided by all means, at least by his former victims (your friend in particular).
His love towards her older sister is also meaningless.
That guy is a pithetic opportunist and if possible, others should be informed of his ations so that he can not continue his actions and victimise more innocent children.
So, my advice is that your friend SHOULD tell her sister the full truth before it is too late.
Wallahu aalam.