Would the questioner be sinful if she stopped correcting the sisters' recitations, and one of them stopped memorizing after leaving the group? And how can she know if she is ostentatious or not, with her constant feeling of worry and sadness and the whispers of self-admiration and ostentation?

Your eagerness for good has increased. Ignore the whispers of ostentation (riya') so that you may continue memorizing the Quran and benefiting people, and do not abandon an act of worship out of fear of ostentation. First, focus on perfecting and completing your memorization of the Quran, then focus on transmitting it and teaching it to people. There is no harm in teaching from what you have memorized, as long as it does not affect your memorization activities. There is no sin upon you if you apologize to some sisters for not completing with them due to your preoccupation with memorization. Try to combine benefits as much as possible, and if it is difficult, prioritize your memorization over others.