Here's some things to back up Eagle:
A) Perhaps one of Brock's siblings was on Ash's back.
B) Perhaps one of Brock's siblings was not there.
C) Perhaps Flint was rounding up when he said Brock had 10 siblings, rather than 9.
Whatever the case, if there are 10 siblings in the Harrison family, you have to understand that it is common practice in Japanese culture to refer to oneself as a part of the group of children in the family. If I have a two sisters and a brother, for example, I would say that there are four siblings in my family, not me and three others. So Flint would refer to his son as one of the group of ten brothers and sisters. This is because Japanese people take more notice of the group rather than the individual. If you still have a problem with this, then I guess you can just blame it on the translation.
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