Taxpaying Adult.
Made/is currently making Don't Take It Personally, I Just Don't Like You and other gay video games to make you sad.
Watches too many horror movies.
What is the best way to tell a close family member you don't want to devote a year and change of your life to working on their app idea that they think is going to be the next big thing?
I've tried "I'm too busy for the forseeable future" and "I'm not sure there's a large enough market for that concept" and even just "That project doesn't align with my skillset" and none of those seem to be working.
The annoying part is that they have enough money they would probably sign it. I would definitely get paid, pretty well in fact, I just straight up don't want to work on this project because it doesn't fit into my wheelhouse and would take up my small amount of free time.
"I don't want to" should be sufficient for anyone who respects you.
If it's a good relationship where you can just set boundaries, do that.
Back when I had to deal with that sort of thing early in my career, though, I'd dismiss it with "that's not the kind of work that I do," and then offer to find them someone who could use the money. That usually ended the conversation, because it established that they're not getting free work, and they're not going to want to tell you that they expected you to do it for them for fun.
