Translation:My older brother is inside.
中 = middle, center (or inside given the context) に = locational particle equivalent to at/on/in (given the context) So yeaps, 中に would most probably mean "inside".
が = topic particle - as far as I know (and I am also a newbie at the differences between が and は) が is specific. So when you've got 兄が, you're probably referring to your older brother specifically.
います is a special verb that would mean "there is" or more literally "to exist". います = "to exist" for animate objects あります = "to exist" for inanimate objects
Thats what I think! :) Just like you, i'm a newbie at this but I am pretty sure thats how it works.