Treat it as unavailable labor until proven otherwise. An 22-year-old child is outside E-2 dependent status, which covers unmarried children under 21, and dependent status would not authorize employment anyway. Projections should reflect wages the business must actually pay, because non-marginality is judged on the enterprise's demonstrated capacity.
Show capacity that does not depend on free help
Non-marginality can rest on present or future capacity to generate more than a minimal living for the investor and family, or on a significant economic contribution. Neither test is a fixed job count. Build the case on revenue drivers, contracts, pricing, and a staffing plan the business can fund.
If the numbers only work with unpaid relatives, the reviewer will see the same gap you do. Show the capacity through drivers a reader can check: units or engagements per month, price per unit, the staff hours each consumes, and the point at which another hire becomes necessary. That structure survives scrutiny better than a growth percentage.
Hypothetical example: a medical writing agency producing clinical evaluation reports plans forty reports in its first year at a stated fee and needs roughly one experienced writer per twelve reports; that arithmetic either supports the payroll in the staffing table or it does not, and it is visible either way. Where the capacity argument rests instead on a significant economic contribution, identify what that contribution consists of and what evidence supports it.