I can't say how she'll react, but the only women I've come out to that was in her 70's was my gran.. And she took it really well, has been just so supportive and kind.. And pushing me to improve quite a lot, always demanding I wear a skirt or dress when I come see her

You may be in for a very difficult talk, but going ahead with transitioning there wont be any option but to tell! Perhaps best to do it some time before you blossom fully, so there is some time for her to adjust.
Best of luck Christine <3