Moving further back down the tree, no-one who is remotely familiar with extant mammals would ever confuse, say, a lion with a wolf, even though they are both members of the order Carnivora. Would it be similarly true that no-one would ever confuse the theropods Tyrannosaurus and Allosaurus?
Tarbosaurus Tyrannosaurus
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Gorgosaurus \/
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Allosaurus \/
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The differences between Tyrannosaurus and its very close relative Tarbosaurus would be very hard to spot based on our knowledge of their bones - the skulls are shaped all but identically, for example. But, like lions and tigers, they might have had completely different skin colouring or patterns.
The differences between Tyrannosaurus and Gorgosaurus would be rather more apparent: apart from being somewhat smaller, the latter is generally more slender, has a narrower snout, a proportionally longer tail and other differences that would be immediately apparent to anyone who was as familiar with these animals as we are with, say, lions and cheetahs.
Going further down the tree, despite the gross similarities between them, Tyrannosaurus and Allosarus are different enough, both in head shape and body proportions, that even at a distance the differences would be obvious to anyone at all familiar with them.
More generally, though, you can't answer a question about how similar members of the same genus, family or whatever are to each other, because there is no general definition of what a genus or a family is - nor indeed of what a dinosaur species is (although most zoologists have definitions of species that they're happy with for living animals.) See ``When is a new dinosaur erected as a new species or genus?'' .)
For example, there is much more variation between the eight species of Psittacosaurus than between the two genera Tyrannosaurus and Tarbosaurus. The whole process of assigning dinosaur species to genera, and genera to families and so on, is arbitrary and inconsistent. Better just get used to it.