IBM years ago came up with a device that would help the hearing impaired to communicate with people with no or little hearing problems. But, it wasn't a mobile (the one that you could carry around with). I don't know if they have one that is mobile now.
You might want to look into
sComm's Ubi Duo.
EDITED to add: I can understand about your mother-in-law feeling uneasy about learning signs. I would feel like the same way.