Istanbul Airport to Sisli

There is no train or metro to Istanbul Airport right now, so train is not an option to Istanbul Airport.
The taxi fare to Sisli from Istanbul Airport will be around 150TL at a no traffic hour. You can check fares from Istanbul Airport to anywhere in Istanbul link below:
https://isturkeysafe.com/New-Istanbul-Airport-Taxi-Fare-Calculator
Also you can take bus to Sisli from Istanbul Airport. The fare is around 20TL. H-2 bus goes to Mecidiyekoy from Istanbul Airport;and Mecidiyekoy is near to Sisli. The bus stop locations and timetable can be found in the page below(in the page, search for Mecidiyekoy).
https://isturkeysafe.com/ISL-istanbul-airport-by-bus