I will offer advice but it will be what you have already received - don't do it.
The business of a franchise can be extremely good with a capable operator - but this isn't the end of the story.
The franchisor can generally "call in" the business at will under almost any pretext leaving the operator with no business but with an on-going lease -if that applies - with no way out and, possibly, a personal guarantee of rent to the landlord.
Franchisors often restrict, or limit, what a franchisee can sell (if a retail point) to what is provided by the franchisor as a wholesaler - which may not be satisfactory to the operator.
Best of luck to your sister but carefully watch any franchise being offered. If she buys, ensure she uses a lawyer knowledgeable on franchises - not just any lawyer or one your sister has known for years or whom her parents dealt with back in the past. There are specialists about. Never, ever, use the lawyer the franchisor recommends - even if they are low cost.
A successful business is attractive to an unethical franchisor as the business can be called-in then sold to another new franchisee across the road - making a profit for the franchisor and with the new franchise using the goodwill of the now closed business which was opposite.
Also, franchisors can themselves fail - leaving its franchised businesses to go it alone. This may not be all bad as the businesses can change their names and tick along happily, ans separately without big brother watching every move.
Further, franchisors have a business (operating and holding the several franchisees) which can itself be sold to a newcomer and which may, conceivably, disadvantage the franchisees collectively and/or individually.
My case rests with more cons than pros.