The 21-year-old bodybuilder shared a simple but sweet "Congratulations!" in the comments under his half-sister's post, along with some heart emojis.
Joseph, who was not in attendance at the wedding ceremony, is the son of Arnold Schwarzenegger and his former housekeeper, Mildred Baena.
While he was born in 1997, the Terminator star and former governor of California did not reveal to his then-wife, Maria Shriver, that he was Joseph's father until 2011, after which she filed for divorce. Their divorce has still not been legally finalized, however, they remain exes.
Katherine's younger brother, Patrick Schwarzenegger, also shared his congrats on both the bride and groom's posts, as did many of the couple's famous friends. Many of Chris' Marvel Cinematic Universe co-stars, as well as some of his Parks and Recreation co-stars, were among the bevy of A-listers who shared their love in celebration of the couple's happy day.