Children of your sister are not your descendants (at least I hope they are not!), although you share a common ancestor with them. All humans share a common ancestor, but not all humans are descended from all other humans.
Crocodiles and dinosaurs (and birds) do share a common ancestor, but that ancestor was neither crocodile, nor a dinosaur.
Children of your sister are not your descendants (at least I hope they are not!), although you share a common ancestor with them. All humans share a common ancestor, but not all humans are descended from all other humans.
Crocodiles and dinosaurs (and birds) do share a common ancestor, but that ancestor was neither crocodile, nor a dinosaur.
Does that make sense?