The news should not come as a surprise considering the personal conflict that was dramatized during the 2016 election between Donald Trump and the Republican establishment, personified in a way by John McCain. Best known for representing the Republicans in their loss against Obama in 2008, McCain served in Congress for 35 years and before that in the military during the Vietnam War. McCain passed away in August 2018. His family purposefully did not invite Trump to the funeral, unlike the Bush family’s funeral observance, a few months later in December 2018.
Therefore, it is only natural that the McCains would offer support to the Democratic candidate for president—especially one running against a Republican president that they continue to oppose for what appears to be purely personal reasons. It should be noted though, that Cindy and Meghan McCain still publically identify as Republicans and by endorsing a Democrat, it may hurt Biden in the long run, considering how the Democratic party is experiencing an internal revolt of its own against moderate establishment types such as the former Vice President.