"My older sister is outside."
Yes, this translation is really weird without giving context. The same topic has been discussed on these other forums:
母と父の間に私がいます - If you scroll down just a little bit, you will see a comment by Zild555090 with the exactly same question. Coincidentally he also has a Japanese wife who told him the sentence sounds wrong, lol
中に兄がいます - Same problem as before... it sounds like they're describing the outside as if it was the most important part of the sentence, and there is an older brother outside. Not a mother, a father, a rock or whatever, there is a brother outside.
As i discussed somewhere else, this translation is grammatically correct and may be "natural" depending on context, but this structure is not what you would use when someone asks you: "Where are you/your brother?"