I´m a multidisciplinary designer with a human- centered focus, I like to create ideas that solve real problems of people at many different levels, specially those that involve exponential technologies, connected objects and smart devices, on the other hand being a big fan of innovation I enjoy building new products that generate business and above all, have fun while doing it.
I love your idea congratulations, now I think they could expand their idea and consider other people who could use the app, not just family and friends could use and share, from my point of view might include young adults living alone and away from their families, often eating or cooking traditional food makes you feel at home and not miss much, on the other hand many of the young adults do not eat healthy for the various activities we do so this app may also work in this territory.
Turning to issues of design, I wonder what is the format of the content, text or video? in some cases read recipes in smart phones is not comfortable because the screen size is small, I think the best thing for this app is to make small tutorials maybe in video format like "Vine" or "Buzzfeed Food" that might make them simple and useful. You could even add a tutorial "how to make your video" on your app.
I wondered beyond of ranking the best grandmothers, there any other reward for them? I find it very cute the idea of putting in contact with the grandmothers and share photos and videos with them, I thought that perhaps a more banal reward could be a bounty system in which each recipe to share get something in return (for example passes theater, cinema, etc.) that could be of interest to them and could get more people to come together and make community.