Translation:Mom has three older brothers.
It means "older brother" but you have to decide by the context whether to translate it as "brother" or "older brother" since in English it often does not matter. With Duolingo the literal translation will almost never be marked wrong, so I suggest you stick with "older brother" or perhaps "big brother" during this course if you want to avoid being sometimes marked wrong for translating it as "brother"
If you write Mother has three brothers it corrects you to say, Mother has three big brothers. I found this very confusing, thinking that 个meant big in this context. It should correct you to say older brothers, not big brothers, just the way it is in the translation on the following page.
We call them counters. :) I figured out from context that it was a counter, because I am familiar with counters from Japanese. But this must be hopelessly confusing for people who are new to the concept. The lesson can do a better job introducing that.
This lesson also needs to do a better job of making clear that the character 哥 means big brother. (This is a limitation of duolingo's lesson structure that I've complained about elsewhere. I am learning a lot of hanzi pronunciation and fewer hanzi meanings... It's frustrating, especially if I'm practicing on mobile and can't view the lesson notes. Totally asinine that they aren't available. It actively discourages me from studying.)