Longbottom -- yes, Snape did know that family. Alice and Frank had seen their end in all the important ways already, and Snape knew they'd had a child, even if he'd yet to meet him -- he shared an important birthday with another child he'd yet to meet. It was coming up soon, their entrance year to Hogwarts, and Snape very much did not look forward to it for so many different reasons.
He didn't say as much, only offered a downward tilt to his lips like maybe he was acknowledging the information he was being given.
Processing it, though, was actually rather difficult. This person before him should have been a little shit, barely a teenager and here he was, adult and from 2007 and it was -- upsetting was the best word for it. "And now you're here." Snape said it carefully -- obviously it wasn't a question, so much as a statement of fact. But there weren't many questions he could ask that weren't incriminating in one way or the other.