Fate sharing in unicast applications means that so long as there is a path that IP can find between two applications, then TCP can hang on to the connection as long as they like. However, if either party fails, the connection certainly fails.
Fate sharing between multicast end points is a more subtle idea. Should 'reliability' extend to supporting the connection for k recipients failing? Clearly this will be application specific (just as timing out on not getting liveliness out of a unicast connection is for TCP - we must permit per recipient Timeouts/Failures).