How do i support my sister in labour?

Distract her by telling her stories of some childhood memories together. Tell her she's doing a great job and that her husband would be so proud of her! She's going to squeeze your hand, so if you have a broken hand by the end of it, don't be surprised ha :) She might yell at you when you tell her to breathe too. It's normal, she'll apologize later! good luck :)