Does Jacob in Twilight saga tell his sister about werewolfs?

No he is not.

He is a shape shifter.

The difference between a shape shifter and at werewolf are these:

A shape shifter is able to control when to shape shift, while a werewolf only changes under the full moon.

A shape shifter can have the shape of anything, in this case a wolf, while a werewolf always looks like a mix between a wolf and a human.

A shape shifter are able to think and control itself while in wolf form, while a werewolf reacts only on instinct and don't / rarely remember anything afterwards.

A werewolf can only be killed, by being shot by a silver bullet, normal bullets have no effect. Silver is the only thing that can harm a werewolf, where as shape shifer might heal fast, but are way more mortal.