According to Sistanis following fatwa;

84. Question: How can we know the time of mid-night? Do 00.00 hours the point of mid-night as it is commonly held by some people?

Answer: Midnight is halfway between sunset and true dawn. So if the sun sets at 7 p.m. and the true dawn begins at 4 a.m., then midnight will be at 11:30 p.m. The criteria for determining midnight are the timings of sunset and true dawn, which differ according to place and season.

My question is: When it comes to Salat al Maghreb and Isha, do we count that the time has passed by Sistanis definition of midnight, or the standard 00.00?

It should be Ayatuallh Sistani’s definition since your question is regarding his fatwa