What is the adjective itong in of big sister?

The adjective "itong" doesn't have a direct English translation, but it's used to indicate something specific and close to the speaker.

So, "itong" in "big sister" would be best translated as:

"this"

The complete phrase would then be:

"this big sister"

This suggests the speaker is referring to a specific big sister, perhaps one that is nearby or already understood in the context of the conversation.