Yeah but what's the actual job? Because swapping a few boards is way different than replacing the whole frame. If you're buying all new composite decking and doing a full weekend of work, that's hundreds in materials and a whole lot of your time. Are people expecting you to eat that cost?
But since when did family start charging each other for basic help? If your brother needed a hand moving, you wouldn't send him an invoice for your time and gas, lol. It's a deck, not a client job, so treating it like a business deal feels kinda cold. Sometimes you just do stuff for people you care about without running the numbers.