Only if the investor is personally liable and the loan is not secured by the assets of the enterprise being purchased. A seller note secured on the roofing company's equipment and receivables is excluded from the investment; the same amount borrowed against the investor's own home or on a personal guarantee without security on the business would count.
The security behind the loan decides it
The rule is about whose money is at risk. If the roofing company fails and the seller can recover only by seizing the company's trucks and receivables, the investor has risked nothing on that portion; if the seller can pursue the investor's personal assets, the investor has. Ask the seller whether the note can be restructured as an unsecured obligation with a personal guarantee, or secured on assets outside the enterprise, and weigh the commercial cost of that against the immigration benefit.
Then look at the remaining consideration: the siblings' own savings and any personally secured bank loan are their investment, and the substantiality test asks whether that amount is proportionate to the cost of acquiring an established roofing business of this size. There is no fixed minimum, but a small personal contribution beside a large seller note will look like the seller is still the one at risk. Adjust the structure before signing, not after the consular interview.