I look at their charts daily and they are always almost identical. Do they need each other to perform properly? Forgive me if this seems like a noob question.
Noob questions are OK, as long as they're honest and not for answers that can be found on this forum with a quick search.
These currencies are linked only by the investors that hold them. A lot of people own Ether and Litecoin, and of course everyone who owns those two own Bitcoin. So when the market turns sour (or appears to turn sour) people unload all three, move it into fiat cash or a more obscure (and more stable) crypto currency.
I'm sure others have great opinions to add.