This is sort of like those logic problems that ask questions like "Can God make something too heavy for God to lift?"
Who can sing at Aretha's funeral? Her sisters are both dead. She hates Patti LaBelle and Dionne Warwick. Whitney's dead. I don't see the family asking someone like Celine Dion -- although she's the only singer who came close in the 1998 VH1 Divas special, shown above. (Watch Aretha bulldoze the rest.)
Adele? Mavis Staples? Chaka Khan? Mariah Carey? Gladys Knight?
Who's good enough to send off the Queen of Soul?