This is a lie. Joseph Biden pardoned Francis W. Biden, James B. Biden, Sara Jones Biden, John T. Owens and Valerie Biden Owens among many, many other dubious choices.
I'd imagine because of the talk from the then-incoming administration talking about how he'll go after their families unless his personal ongoing charges were dropped [0]. I would do the exact same thing if a vindictive POS was about to gain power.
[0]: "Trump threatens to prosecute Bidens if he’s re-elected unless he gets immunity" https://www.theguardian.com/us-news/2024/apr/30/trump-second...