searching for a children's fantasy series - east asian siblings, black liquid in pots involved

I only remember snippets from the book. If someone could help me find it that would be much appreciated.

I feel like the children were Japanese but I'm not sure about this. The main was an older sister, probably around 12, and the other main was her little brother.

I remember:

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  definitely something about a car ride and the little brother picking his nose and eating it, getting called out by his sister; this is near the beginning of one of the books

  
  


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  something about ending up at the bottom of a waterfall and being cared for by orcs or something

  
  


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  the main is a swimmer and upset about puberty (boob development) because this will create drag

  
  


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  something about black, oily liquid in a pot that worked as a portal. an illustration of this also appeared on a cover of one of the books

  
  

This was definitely published before 2010.