Translation:My mother has ten siblings.
Yes this is actually a very important issue the ga is a partical in Japanese similiar to wa/de/ni which have been used all over the place in these lessons. They conote context of sentences by denoting the way the works are being positioned. It is similar to not knowing the difference in English of knowing if the cat is on the box or the cat is in the box.
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.
Thats a big family... But aside from that, could you also say "I have 10 uncles" or is there some honor thing to uphold by not bragging about family, and also distancing yourself verbally from them if they are bad or they think you are bad. Sorry for the rambling on, my mind goes weird places when it comes to Japanese honor.