Yes, it's true. Elizabeth Sheridan is Jerry's mom's name in real life, and she now lives in the Valley, but back in the 50s she and James Dean lived together in New York. Only problem was that Jimmy also had a homosexual lover who was older, richer, and could introduce James to the right people. JD did have a full-fledged sexual relationship with Elizabeth (her friends call her Dizzy), but he was leading a double life.
The article with this info is in the Los Angeles Magazine, April 1996.
DJB