Soon-Yi was adopted in 1978.

Farrow’s marriage to André Previn ended in 1979. In 1980, Farrow began a long-term relationship with filmmaker Woody Allen.[11] Allen later adopted two of Farrow’s adopted children: Dylan Farrow and Moses Farrow. In 1987, Mia Farrow gave birth to Ronan Farrow.

So legally not her step-dad. Still pretty skeezy.