My sister is chronically late and I really wish she would show up on time for me.
Do you know why your sister is chronically late?
She's terrible at time management.
It's understandable to be frustrated with your sister's lateness when you're depending on her. Have a conversation with her and explain how her actions are inconsiderate to others. She may not be aware of how much people are upset with her chronic lateness.
Actually I probably shouldn't be upset about this because she is at least better than her rude and ignorant husband and children.
That's unfortunate that her husband and children are that way, however it's unfair to you that your sister is always late and not taking your time into consideration. You're allowed to be frustrated by this tardiness and it's worthy of a conversation.