Talking...

Okay so my son will be 19mo on the 19th and he babbles A LOT, but as far as talking goes he only really says mama and dada - if I say dog he looks at the dog but won't say it same for the cat he actually makes a meow sound. Anyhoo, I googled this and I'm freaking out that he's not talking yet (not that we understand) his sister has autism so shes doesn't really talk (recessive expressive language disorder) she can just not a lot. Please tell me I shouldn't worry