Translation:My mother has ten siblings.
Your sentence changes the subject to "my mother's siblings," which isn't exactly what we were asked to translate. It would kinda be like, "10 siblings of my mother exist" instead of, "My mother has 10 siblings." It gets the same meaning across but it isn't what we were meant to translate and it sounds kinda odd.
Answers with "9" should be accepted. Japanese people have been telling me for years that in sentences like this, the person in question is included. Not only have they been telling me this, but they demonstrate that that is their thinking; if you ask them how many siblings they have they will say, "three, an older brother and a younger brother," for example.
Ah, so it really is: "As for my mother, there are ten siblings" i.e. there are ten siblings in the family (not ten other siblings)?
After a little more research, it turns out the "not including the speaker" thing applies to a different sentence structure, using ～です rather than ～います.