Yes, having family members work for you for free is legal.

My accountant once encouraged me to try to get my family members involved in a business I ran just for that reason.

I think the question of age comes into play though.

Possibly. But there are an awful lot of very young family members working on commercial farms.

That’s why rural states give drivers licenses to kids as young as 14 — so they can drive a tractor between fields.

8yo me would have been intrigued by this possibility. (This was on a farm.) As an adult, it seems that anything kids might do around the house, they might do in a family-owned motel. If there are laws against that they are bad laws and also inconsistently enforced.

There's a family business exception.

Underaged labor is legal in the USA as long as they are employed by their parents and the work isn't in manufacturing, mining or occupations declared hazardous.

https://www.dol.gov/whd/regs/compliance/childlabor101.pdf

You could make the argument that startup founders who aren't taking a salary from their early stage startup are also performing "free, illegally so" labor.

I think in that case, you can't work for yourself illegally.

If the startup is incorporated, then you're working for the corporation, not for "yourself."