Rephrase, "many people [with MODY-2 diagnoses] have family histories of early heart attack." This is where the scientific method comes in. Human beings are notoriously bad at diagnosing patterns, or their absence, from anecdote. It is quite plausible that MODY-2 carriers might have elevated risks of heart attack, or other complications. But that's an impulse to try to study the topic more rigorously.
In the study we're discussing, which seems to be the best evidence we have on this topic (though there are other studies on it, too), the rates of heart disease among a pool of 99 MODY-2 carriers were effectively identical, actually slightly lower than, the rates of heart disease among a pool of 91 family members who do not carry the MODY-2 gene mutation. This despite the fact that the GCK-MODY carriers have meaningfully higher average blood sugars, as expected, than their relates who do not carry the gene mutation, and by definition they've had those higher blood sugars their entire lives, since before birth.
Yes, these folks, both the gene mutation carriers and the non-gene mutation carriers, have a spectrum of blood sugar levels, some of which we'd categorize as pre-diabetic. But that's the point. What the study is doing is a "natural experiment" where family members who are presumably otherwise similar (other genes, lifestyle, diet, etc.) but who differ on one crucial variable (whether they have the MODY-2 gene mutation or not) are being compared. And the study strongly suggests that having the MODY-2 gene mutation doesn't increase heart disease risks, despite the life-long elevated blood sugars that gene mutation entails. In fact, if there is a difference in the population, if anything the gene mutation may lower the risk of heart disease. The prevalence of heart disease is too low for the comparison to be statistically significant at these sample sizes, i.e. we can't be sure that gene mutation carriers have a lower, or an identical, risk of heart disease as non-gene mutation carriers, controlling (via natural experiment) for other relevant factors, but it's hard not to be at least somewhat reassured by the results.
And for what it's worth, I really welcome @Jenny's engagement, because it's a chance to push myself to try to articulate what I think, and where we may agree and disagree. So I hope we can continue spirited dialogue without it becoming personal, because I think that serves us and the broader community on here.