The way the aunt told the story as in the OP leads to believe he is the fruit of betrayal but still adopted as a son.
It’s still second hand information as I understand it was long time ago and probably it was hushed (aunt is related to WW or BH? That could give some hint of how attendible the information is, if the WW told her "I got pregnant from the AP" is less a speculation).
They kept secrets and the man who was unwillingly connecting you with this fallout, your husband, passed.
If he was alive he may have had stakes and the right to know and find out the truth.
You have no ties with the brother, so you are carrying the burden of a secret from an unfaithful mother, but it’s not that you are the one hiding it or perpetuating it.
You feel the weight because this knowledge changes the light over the reality of the man you love (present tense because he passed, but it feels like you still have love feelings), and a close person he probably loved (his brother) realizing they have been living in the shadow of a big lie.
A lie that was probably out of love for both R and the WW, but still a,lie. And truth has the remarkable trait of always surfacing even decades after, it can be found because it is the lies that have an expiration date.
Think about it this way: there’s no dishonesty or connivance with the lie in you.
You stumbled upon a truth about a person who is not present in your life, is a very distant acquired relative that doesn’t seem much interested in acknowledging your existence.
In a different setting and stage of life this information would carry more weight as there can be borderline emergencies where you need to know for sure if two people are blood related, think medical emergencies.
This is apparently no longer a concern, you unveiling this secret to a distant person would be more akin of publishing out on social media than really addressing an important issue with a close person (he chose not to be close).
There’s the other side of the psyche that we don’t like uncertainty, so not knowing the answer to this secret is likely unpleasant for you. But for him and his family could turn out to be reality shattering.
I say that the best decision and the only option left is to ask the aunt to be the bearer of such revelations, as she is in closer contact. And you keeping this knowledge to yourself unless he comes one day demanding answers, in which case you can tell him the truth, he will be then in charge of his agency and not oblivious like he is now.
It sucks but it has o power over you, I wonder if it didn’t awaken some questions or doubts from your distant past as you were betrayed before.
The ripple effects can make you wonder which might exist from your past that you don’t know off.
What I say it could have been a minor trigger, that let you wondering, you may try to address those emotions if that’s the case, to put them to rest and return to the peace without feeling the weight of this burden as much as it did when you learned about it.