It's very kind of you to investigate this and do your relatives' due diligence for them -- I hope they'll accept with grace the fact that they're being scammed. Disney offers no such service (free FP+ for every attraction) for purchase or resale by outside agencies. The only way I can see any tour group offering something
like this is if they were hiring out a "guide" with a GAC (the most recent incarnation of the disability assistance pass) and using that system for themselves and their party (i.e., their clients). Even then, the group wouldn't be getting Fastpasses, per se, but return times to come back to the next attraction
at roughly the same time they'd be riding if they waited in the standby line. On their return, they could then use the Fastpass line -- meaning, they'd technically skip the line for that attraction, but not with the immediacy of an actual Fastpass, and they could only have one such "return time" pending at once. They'll be twiddling their thumbs or visiting another attraction (standby only, during the busiest week of the year!) in the meantime... not to mention contributing to the fraudulent "resale" of GAC services, and perpetuating the kind of abuse that led to the discontinuance of the original DAS system...
If your cousin and SIL enjoy throwing away money like this, however, please tell them I've got some amazing bridges I'd like to sell them...
In the meantime, they should check out Disney VIP tours, which are legitimate (and incredibly costly -- but at least they'd be getting something in return, instead of throwing good money after bad) and do offer expedited access to all FP+-enabled attractions.
https://disneyworld.disney.go.com/events-tours/vip-tour-services/