I’m not exactly certain what name Grandma used to refer to her sister-in-law, but it always sounded to me like “See Dee Yuh” or something very close to that with the second syllable merged into the final one. There are some key concepts there: what Grandma actually said and what I actually heard. My interpretation of what she said may have been incorrect. The fact that Grandma only had a lower set of teeth might have made an impact as well.
In hindsight I should have asked Grandma how to spell it. She would have known and she would have told me. I never did ask, but that would have answered the question then and there. In some families the answer might have been “I don’t know, I never had to write it down.” But Grandma would have known how to spell it of that I’m pretty certain.
After all, she learned how to spell Trautvetter at a young age so Celia should not have presented a problem. Pronouncing it was another matter entirely.