This is "Foundationed Deep," a feature where we look at particular odd/strange/interesting instances of retroactively connecting different comic book characters (for instance, Uncanny X-Men #268 retroactively established that Wolverine knew both Captain America and the Black Widow from World War II). Today, we look at how Havok first met Mister Sinister!
As you likely know by now, Mister Sinister is, like, super-duper obsessed with "Summers DNA" and with the idea of combining Summers DNA with Grey DNA to form a super-mutant who could destroy Apocalypse (who Mister Sinister has been serving since the late 19th Century and who he was now so sick of he wanted someone else to get rid of him).
In X-Factor #39 (by Louise Simonson, Walter Simonson and Al Milgrom), we learn that Sinister specifically had Scott Summers and Alex Summers split up as kids so that Sinister could study Scott Summers closely...
In Classic X-Men #42 (which came out roughly around the same time as X-Factor #39), in a story by Chris Claremont, Mike Collins and Joe Rubinstein, we saw some of Scott's time at the orphanage in Alaska where Sinister studied him. Now, do note that this story was written back when Claremont intended for Sinister to be a kid who could never grow up but had the ability to create other forms that could serve him, so the big old scary "Mister Sinister" was just this kid's idea of what a super villain should look like. Since Claremont never got around to actually outright establishing that in the comics, the current explanation for these stories is that Sinister is using his powers to shape-shift himself into the little kid, Nate, who was at the orphanage with Scott Summers, so Sinister could keep an eye on him while he experiments on Scott at night.
Sinister was fine with Scott being bullied, as he wanted to see how Scott would react, but when one bully went too far and began to interfere with Sinister's operation, he had him eliminated...
A new doctor at the orphanage, Robyn Hanover, took an interest in Scott Summers and introduced him to a pilot couple that she knew. They expressed interest in adopting Scott.
Sinister couldn't have that, so he captured Hanover and brainwashed her and turned her against Scott and then had the prospective adoptive parents killed...
Super twisted, right?
Anyhow, Scott eventually ran away and lived on the streets until Professor X took him in and he became a member of the X-Men. Seemingly the first (until a retcon established that Jean Grey was actually the first).
Okay, so it seems like Alex got away from the Sinister stuff, right? WRONG!