Ask HN: How should the idea-person behind a startup be compensated?

A while back my sister-in-law approached me with an idea she had - or better said, she was asking why there wasn't a service that did X. After some investigating, and looking in to the feasibility of it, and figuring out possible routes of monetization, and finally building out a business plan I now have been working on the technical side of things and want to launch shortly. I am pretty sure this concept will be successful, it provides a great service to people - it's not just something I think they want, but at this point know users want.

The big question is, I don't know how to split equity on something like this. How should the idea person be involved? Does she get a founder status even though (and I don't mean this in a bad way) she can't contribute in any other way? I want her to get something out of this if it's successful, and it's not about greed or anything like that - but what is fair, and what won't scare potential investors away if they were for example seeing a non-founder with shares?

Thanks!