FWIW, I've seen people who were not Asian call their in-laws "Mom" and "Dad." It just depends on what they asked to be called, I always assumed. I don't have in-laws but my parents always called my grandmothers by their first names. The father-in-laws were dead either before or really early in the marriage but it would have been the same.
I think it is usually a case of, "we're all adults and we're family, so first name basis seems right."
