He did not.

A. She was not underage. She was 20 when Mia found the photos. She had just returned from her first year in college. Allen never really spent any time alone with her prior to that.

B. She was not his stepdaughter. Soon Yi Previn was the adopted daughter of Mia Farrow and her then husband Andre Previn. Woody and Mia were not married and were not living together.

You can mention the lines he did cross but those are the facts.

As for Dlyan Farrow, this was looked at pretty much as soon as it was alleged. And it the multiple doctors who examined he said no abuse occurred and that is why he was not charged.

This is by no means whatsoever a cut and dried case.

Unfortunately we know that if you have a 7 year old tell you a story over and over and over you can actually imprint a memory.

https://en.wikipedia.org/wiki/Lost_in_the_mall_technique

So I think you have to go back to original events. This allegation happened after the Soon Yi affair was discovered and during Farrow's and Allen's custody battle. That is the backdrop.

This is a good timeline of those events https://www.thenation.com/article/woody-and-mia-modern-family-timeline/

January 1992: Mia Farrow finds intimate Polaroids of her (adopted) daughter Soon-Yi Previn, age 20, under a box of tissues on the mantle in Allen’s Manhattan apartment. After confronting Soon-Yi and confirming the affair, she tells all but the youngest children what she has learned.

The incident with Dylan was said to happen in August, two days before they were sign their custody agreement. There is a lot about Mia Farrrow's behavior at this time that had observers concerned

Among the folks who either say no abuse occurred or raise significant doubts are the following

• Dr. Susan Coates, a child psychologist, who was working with the family for two years.
• The pedatrician Farrow takes Dylan to, the day after the incident. When asked where her father touched her she pointed to her shoulder.
• The doctor's who examine her 5 days after the incident
• Dr. Nancy Schultz, Dylan's therapist
• A group of child abuse experts at Yale-New Haven Hospital investigating the abuse allegation for the Connecticut State Police. The interview her 9 times, their statement

describes inconsistencies in Dylan’s accounts: “Those were not minor inconsistencies. She told us initially that she hadn’t been touched in the vaginal area, and she then told us that she had, then she told us that she hadn’t.” In interviews, he testified, she had “a rehearsed quality,” and once said, “I like to cheat on my stories.”

“We had two hypotheses: one, that these were statements that were made by an emotionally disturbed child and then became fixed in her mind. And the other hypothesis was that she was coached or influenced by her mother. We did not come to a firm conclusion. We think that it was probably a combination.” But, he stated, “We don’t have firm evidence that Miss Farrow coached or directed Dylan to say this.”

Leventhal noted one “very striking” consistency in Dylan’s accounts of the abuse: she connected it with “one, her father’s relationship with Soon-Yi, and two, the fact that it was her poor mother, her poor mother,” who had lost a career in Allen’s films.