Steve wasn't a stupid man, but he wasn't used to the concept of children coming back from the future to meet their parents. (He was still new to this universe. It was going to take a few moments for things to click. Why would the kid be so evasive about his last name? Rogers was a perfectly fine surn-- ohhhhh.)
He wanted so badly not to scar the boy with his reaction that Steve sort of... failed to react. He stared somewhat blankly as he tried to process.
Loki trick? No. It wasn't big and obnoxious and deadly enough. Besides, where was the endgame? Steve was not the obvious first Avenger to take out. That was Banner, or Thor, if the big guy had come through the Vortex, but--
Okay, not Loki. Uhhh...
...Steve didn't have an explanation. "Sorry," he finally said aloud. "So we're related, then?"