It's super convenient having one sister who is also my best friend.

• I can say, "My best friend is my sister," and no one can ever get mad or jealous, because that's adorable.
• Instant future maid of honor. No tough decisions.
• Having just one means I don't have to choose between them.
• It's useful to hang out with her because then I can ignore a lack of a social life.
• I can do and say whatever I want around my best friend because she is biologically forced to love me unconditionally.