Translation:Tomorrow we are going to go to my sister's house.
Yes it is correct in American English.I would say that it is becoming or already is a future auxilliary marker especially in its spoken form. "I am going to go to the store" is pronounced "I'm gonna go to the store" which always denotes a future event. "I am going to the store" can describe the present or the near future. But never is the spoken "gonna" used to denote motion. "I'm gonna the store" is never said. "Gonna" is a spoken future auxilliary marker in many sentences. "I'm gonna beat you" " He's gonna catch the ball" etc. This is a fascinating example of how language evolves. The purists will say that this is just laziness but what some call laziness others call economy of speech.
There's a subtle difference in English usage of "will" versus "(to be) going to." Really. Just google it!
Examples include the use of "going to" for some action that has not yet happened but is already planned.
'What will you wear tomorrow?' or 'What are you going to wear tomorrow?' ... 'I'm going to wear my new dress.'