The Logan Lucky star, 31, described her younger brother as “pure light” after he took his own life on Sunday.
The siblings are the grandchildren of Elvis Presley and the children of his daughter Lisa Marie Presley.
Sharing a string of photos of them together, Keough wrote: “Mornings are the hardest. I forget you’re gone. I can’t cry because of the fear that I will never stop. A pain that’s new to me.
“You. There are no words for you. Angel is the closest I could think of. Pure light. Baby brother. Best friend. Wild man. Intellectual. Witness to my life. Twin soul. Protector. Too sensitive for this harsh world.
“I hope you give me strength to endure the giant hole you’ve left in my heart. I hope you give me the strength to eat. I hope you’re cradled in love. I hope you feel my love. I hope you feel god.
“You are god. I can’t believe you’ve left me. Not you sweet Ben Ben. Anyone but you. I guess this is true heartbreak. I hope we meet again.”
Her mother Presley also has twins from another marriage.
Sarah Paulson was among the stars sending condolences, writing: “So deeply sorry for this loss. Sending you strength.”
Laura Harrier wrote: “I’m so so sorry Riley, sending you so much love.”