Is the supplication made by the girl's sister answered?

Speaking with or corresponding with a non-mahram woman without necessity is a gateway to tribulation (fitnah). Therefore, it is impermissible to speak with a young non-mahram woman without necessity, as it is a cause for temptation. One must repent to Allah by ceasing the sin, regretting having committed it, and resolving never to return to it. Repentance erases what came before it.