What's the meaning of bellow proverb? (English is not my first language) "You can't have your cake and eat it"

You can't have it both ways. E.g. If your sister can't cook but lets you watch what you want you want on tv when babysitting, you can't have it both ways that she can cook as well as being relaxed about what you watch on tv.
*also it's the below proverb. Not many people use proverb, most peopl would use saying