When my sister leaves to go to work in the morning her cat always cries... Does anyone know why?

I think it's really possible that your sister's cat always cries when she leaves for work because it's lonely!

Although cats are quite solitary animals, they do need some interaction, just like we do! So when your sister goes off to work, her cat is probably sad at being left alone, which is why it cries. That sound can signify that they're unhappy with being left alone, and there are other reasons that go hand in hand with this:

• Stress
• Cry for attention

Ask your sister to watch out for these other signals that her cat is lonely:

• Destructiveness. If she's coming home to boxes, cushions or furniture shredded, this can be a sign of boredom and loneliness.
• Aggressiveness. If your sister notices that before the cat cries he or she is being pretty aggressive towards her, this could be another sign.
• Over-grooming. Watch out for excessive fur-pulling or over-cleaning, because this can show they want more attention.
• Spraying. If the cat is marking your sister's home with urine of faeces, this is a way of it trying to get your sister's attention that it doesn't like being left alone when she goes to work.

You might want to have a look at this page about cats that cry/meow excessively: http://www.petsadviser.com/behaviors/cat-meows-constantly/