This depends on if the time of Isha is due or not. In some countries – such as Saudi Arabia- they leave a fixed time between Maghrib and Isha of one hour and 30 minutes. It is known that that Isha time is due about only an hour. Therefore, if a person prays Isha half an hour before the Athan, this is OK. I don’t know when Isha is actually due in the sister’s country. This is known by looking at the direction of west after sunset. There would be a redness at the horizon. When this redness disappears, it is time for Isha.
And Allah knows best