Is the truth always the best?

What makes me ask this question is a very profound truth which I know about my wifes real father – her sisters and brother are really only her half siblings.
The annoying thing is her mother told her that her dad wasnt really her dad when she was a teenager, but at the same time didn’t tell her who her real father was.
Now her daughter in law is going to have a baby boy soon and they intend to name him after his “great-grand-dad”, although that wont be true.
Another factor is that my wifes mum always takes the high moral ground on everything and pontificates ( which makes me fume inside ), and is very bossy. She’s now in her 80’s
Would you let the truth out or leave well alone?