It was actually her cousin! Edith (Alec's mother, for those of you who haven't heard the name) was sexually abused about three or four times by her cousin Leopold when she was 9/10 and he was 14/15. Leopold would corner her at family gatherings and kept her quiet by heavily putting the blame on her and telling her she'd face dire consequences if anyone found out.

This is the same Leopold who went on to father Bertrand and Alec years later, during Edith's marriage to his brother Richard. This time the sex was...dubiously consensual? Edith, having strictly denied herself any form of sexual pleasure or freedom during her teenage years as a result of the abuse and the religion she used to cope with it, was very repressed by the time she got married. Leopold had forced pleasure on her before, and so she associated it with him, and when that dam eventually broke he was the one she went to. The affair stopped after she got pregnant, and Edith transferred all the guilt of the childhood abuse and the adultery onto her resulting offspring. This created the upbringing that fucked them over so badly.

Really, it's like a flowchart.