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.
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
something about ending up at the bottom of a waterfall and being cared for by orcs or something
the main is a swimmer and upset about puberty (boob development) because this will create drag
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.