In The Deathly Hallows chapter 1, Voldemort "congratulates" Bellatrix on Lupin and Tonks's wedding.
"I'm talking about your niece, Bellatrix. And yours, Lucius and Narcissa. She has just married the werewolf, Remus Lupin. You must be so proud."
However, there is no way I can think of that Voldemort would know they got married. Even Harry didn't know.
"You got married?" Harry yelped, looking from her to Lupin. "I'm sorry you couldn't be there, Harry, it was very quiet."
The only people who would probably know are a few of the Order of the Phoenix. But how would any of them tell Voldemort? Another explanation could be that Narcissa or Bellatrix would be informed as they are part of Tonks's family, but they claim that:
"She is no niece of ours, my Lord," she cried over the outpouring of mirth. "We - Narcissa and I - have never set eyes on our sister since she married the Mudblood. This brat has nothing to do with either of us, nor any beast she marries."
So how did Voldemort know when someone like Harry Potter didn't?