Here, from 1993's X-Men #23, is the scene where Mr. Sinister lets slip that Cyclops may have more family than he thought:
At the time, it could have meant anything. In 2006, when Vulcan was introduced, it raised the question of how Sinister could possibly have known. Then, in 2019 with the revelations that House of X and Powers of X brought, it made perfect sense that he'd have had that information. So it seems there's some new twist to the story every 13 years; can't wait to see what we learn in 2032!
I think my favourite bit from that sequence is just how polite Mr Sinister was. "Excuse me?" "I'm sorry, did I?"
That's the kind of language that should really set you on edge when dealing with a guy like him.
Adam X has been canonically revealed to be a summers brother too in the X-Men legends series.