The potential problems are communication - does her Grandma understand how Airbnb works, does what you offer fit with her needs? Will you be communicating with the granddaughter as a go-between? (this can get frustrating, playing telephone to get arrival times, etc)
And reviews - if they do leave one, it's not really from the granddaughter's direct experience.
It's also against Airbnb's TOS, so if you end up in a bad situation they may not back you up as much as with a regular reservation.
I have allowed a third party reservation (a local business hosting consultants). It's going to come down to what your gut feel is on the situation.