Why is my sister so insecure?

I'm not trying to be mean or anything, but my sister tries to make me feel insecure about myself.
One time, I was showing my mom a bikini I got. I turned around, and my sister was like "Oh my gosh! Your ass is hanging out." Then my mom went "No it's not.." I looked at my sister confused. Then I turned around, then she told me my boobs were hanging out. I said "I have no boobs!" My sister was like "Yeah, that is really true.." Huh? But then she would say I have no ass..

Then today, I was talking about the cons of working retail. Then my sister was like "You just hate people!" So I was like "What? I love the work environment I was at! I was a Cashier, and I only hated rude people." Then she scowls at me, so I just almost got a bit frustrated.

I can go on, but I don't know why she is trying to put me down.
Every time I'm around family, she says I have no friends. Yet she is the one who hangs out with her boyfriend 24/7 while I am hanging out with different friends.

I asked her why she is insecure, and she just said I don't know
Why is my sister so insecure?
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