"My brother and sister are going to Japan."
Translation:Saudara dan saudari saya pergi ke Jepang.
In English, "are going to" can have two different meanings, one semantic and the other pragmatic. The literal, semantic meaning is that they are currently in the process of traveling there as you are reading this. The pragmatic meaning is that they plan to go there in the near future.
Ergo, because of this pragmatic use of "are going to," another correct way of translating the verb part of the English sentence into Indonesian is "akan pergi."
For another example, if I say, "I'm going to clean my room," that can mean "I will clean my room, but I haven't done it yet." And I may not yet have even departed for the purpose of cleaning my room when I'm saying this.
This is another one of those ambiguities of English.