The 19-year-old midfielder netted just a few moments after coming on as substitute and with the clock on 74 minutes he struck in front of the Gallowgate End.
After the game Longstaff told Chronicle Live: "It as literally my first touch.
"I couldn't believe it but it was fantastic for me.
"When it hits the crossbar it always looks a bit better.
"To be playing on the field with your brother is one thing but for him to set me up for a goal like that is incredible.
"It's not often you get to play with your brother at St James' but when you score like that you just wanted to grab him.
"But Jamaal Lascelles picked me up like a little kid."