'Not all adulterers are found out'
'Not all second marriages end up worst than the first'
You're right of course Pink, but OP is on the brink of a huge decision, and needs to understand what could happen and what she stands to lose.
It could all work out perfectly for her, but it would be foolish to ignore or fail to plan for the worst.
Posters aren't being cruel when they highlight the negatives, and some are also speaking from experience, so it's worth listening I think.
I have a friend who is a counsellor. She tells me that adulterers are always shocked when they're found out. Either they didn't consider it as a possibility, or they thought they would end things before it came to that. I always remember her telling me about a woman who described the shock when her bubble burst and the real world came crashing in - apparently, like a naughty child, she kept saying she wanted to 'take it back'.
So when I advise OP to consider what she'll do when her DH discovers the affair, it's a genuine effort to sharpen her mind, not to be unkind.
OP has asked for advice and posters, some of whom have direct experience, would be remiss if they didn't present a realistic view of how this is statistically most likely to pan out.
As an aside, if your late relative's affair was 'never found out' how do you know about it?