Question: what are the advantages and disadvantages of having a sister? 

Answer: There are very few disadvantages except maybe that you'll have to share some of the spotlight from your parents with her.  The biggest plus is that you will not be alone in the world.  Your parents won't be alive forever and genetically speaking, there is no closer related person to you in all the world than a direct sibling.  You are each half of your parents put together and there is no one else like you.  I think siblings are very important even though you don't always realize how important they are to you until you are grown up and each living separate lives.  You will learn a lot though from a sibling and be able to teach them in return.  I bet a lot of people consider their siblings to be the most rewarding relationships in their lives.
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