Is it obligatory to make up an unknown number of days from Ramadan, or an entire month as a precautionary measure, due to drinking water after the dawn adhan during the past 15 years, believing that the adhan time for imsak was incorrect? And is it obligatory to inform the sister who followed the same timing?

If you drank after the adhan and confirmed that dawn had not broken, then you do not have to make up (the fast), because the basic principle is the permissibility of eating and drinking until the true dawn becomes clear. And the fast of one who ate or drank while doubtful about the breaking of dawn, and it did not become clear to him that it had broken, is valid.

However, if you were unaware of the time or lacked experience with the signs of dawn breaking, then it appears that you must make up (the fast), because the basic principle is that the muezzins are trustworthy regarding the time, and it is not permissible to consume things that break the fast after their adhan unless there is proof that he called the adhan before the time.

In the case of making up (the fast), you must make up the days that you predominantly believe occurred, and inform anyone who followed you in drinking after the adhan.