There is no fixed percentage, but the invested amount must be substantial in proportion to the total cost of acquiring the business, and loans secured on the business assets themselves do not count as the investor's own risk. Personal funds and loans secured on the couple's own assets do count.
What counts toward substantial
Substantiality is proportional: the committed capital is compared with the actual cost of purchasing or establishing the enterprise. The lower the cost of the business, the higher the percentage of that cost that ordinarily must be invested; a very expensive enterprise may qualify at a lower percentage because of the sheer magnitude committed. The source of the capital also matters.
Hypothetical example: the Lake Country couple bring the proceeds of their Canadian rental property, borrow a further amount against their own Lake Country home, and ask the seller to carry the balance secured on the inn; the first two amounts are at their personal risk and count, while the seller-carried portion secured on the inn's own assets is not the couple's risk and does not count toward substantiality. The nephew's contribution counts toward the enterprise but not toward the couple's investment. Government fees are set by the Department of State for the visa application; professional and closing costs are separate.
Ask the seller for a full breakdown of what the price buys so that working capital and inventory are visible in the calculation.