His skin colour is relevant because others of his skin colour suffer disadvantage and abuse every day because others have a problem with it.
Racial bias in (not) awarding the Medal of Honor is believed to have been rife in earlier times. Since the 1990s successive Presidents have made attempts to correct this. His sister may be right it wasn't a factor in this particular case. However until this upgrade is confirmed only 27 Medals of Honor have been issued for actions since the Vietnam War ended, none to African-Americans - I admit it may be possible that no other African-American deserved it. On a population basis it would be 3+.
No African-Americans were awarded the medal in WWII - President Clinton upgraded previous awards for seven in 1997.
In 2005 President Bush 43 presented the Medal to Korean War veteran Tibor Rubin after an official inquiry determined he hadn't received it because he was Jewish. "in affidavits filed in support of Rubin’s nomination, fellow soldiers said their sergeant was an anti-Semite who gave Rubin dangerous assignments in hopes of getting him killed."