Yes, Sling TV customers can use their Sling TV username and password to access the WatchESPN app and WatchESPN.com. Sling TV customers get ESPN, ESPN2 and ESPN3 with the Sling Orange service. Sling TV customers who add on the Sports Extra get SEC Network, ESPNU, ESPNEWS and ESPN Buzzer Beater on both Sling TV and the WatchESPN app.
For more info on the Sling Orange, go here.
For more info on the Sports Extra, go here.
For more about WatchESPN, please visit the ESPN site here