Facetime

Hi there I've recently got my DEAF mother in law who live 600 miles away a iPad so she can keep In touch with her daughter. When we facetime her why is there not the ability to use text as she is DEAF. We started using a white board for hear to read our replies. Since then we've worked out that if I send a sort message in iMessage from my phone as I'm facetime from my iPad it appears at the top of her screen while were face timing her so she can read our replies and ask her questions. Why has Apple not thought of this for the disabled hard of hearing. I have been told that skype has this function so could Apple not do something similar.