Baena, the 21-year-old son of Arnold Schwarzenegger and former housekeeper Mildred Baena, commented on one of Katherine's wedding photos on Instagram with a heart emoji and a simple "Congratulations!"
Joseph, a budding bodybuilder who greatly resembles his father, follows Katherine and her siblings Christopher and Patrick Schwarzenegger on Instagram, but none of Maria Shriver's children follow him back.
Joseph was born in 1997, but Arnold didn't inform then-wife Maria Shriver of his paternity until 2011.
Katherine previously came under fire for not including Baena in a National Siblings Day tribute.
At the time, she responded to a commenter, "Saw in your profile it says 'god always'. Just wanted to remind you of Gods presence in all of our lives and even in yours as you come spreading hurtful words and judgment. He does not judge you or any of us. I am praying for you and sending you love not because what you said is okay, but because I believe you need love to heal whatever is causing you to post comments like this. god bless you!"