Where did the story of Su Shi and Fo Yin come from?

There is no record, pure and simple folklore.

To be sure, there is no Su Xiaomei in history, and Su Shi has only one younger brother, Su Zhe.

The story you told doesn't seem quite right.

On one occasion, Su Shi and Fo Yin chatted. Su Dongpo suddenly asked Fo Yin, "Do you know what you remind me of sitting there?" Fo Yin said he didn't know. Su Shi said, "Master, you sit there like a piece of shit." And smiled. Fo Yin looked at Su Dongpo and said, "Su Shixue, you sit there like a Buddha."

The front and back are the same.

Su Shi can't be so vulgar, huh?