I'm not going to tell my wife that her brother relapsed before he died.
Is that how you want your wife and her family to remember him?
He relapsed because he did not want to listen to my advice on how to get his life together.
I think it is okay to keep hurtful truths from your partner. You don't want your wife hurt anymore than she is already. I think it was good for her to know that he died trying.
I don't want her memory of her brother tarnished.
That's perfectly fine. Keeping hurtful things from the people you love is fine.