Crying over imaginary sister, what is wrong with me?

First of all I'm M/17. Yesterday I had a dream in which I was sad that I didn't have a sister, but then I realized in the dream that I have one. In reality I have no sister so since I woke up I can't stop crying about the fact that I never had a younger sister and I feel like I miss her even though she does not exist but In my heart she exists and I just wish she was real. I know it sounds really weird but I always wanted a little sister and now I don't know what to feel anymore. Is this normal or am I mentally ill? Thank You
Crying over imaginary sister, what is wrong with me?
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