Older siblings are great mentors because they are with you your whole life. You know them so well, so it will be easier for them to relate to you, and get on a deeper level of connection and understanding. They also have lots of experience with things that may very well happen in your future.
Unfortunately, I can not answer this from personal experience. My older brother is a brat. But anyway, apparently older siblings have been on the earth longer than you have, and they have dealt with more things if that makes sense. So apparently older siblings are supposed to be helpful creatures who help you though life and give you advice about stuff.
