I have not been able to find any articles or publications that argue for Nakhtmin being Ay's son-in-law rather than his son. If you know of any resources, please let me know in the comments so I can read them. This concept of Nakhtmin the son-in-law comes from only one book as far as I know: Michelle Moran's popular historical novel Nefertiti.
I'm not criticizing Ms. Moran for portraying Nakhtmin and Mutnodjmet's relationship as a romantic one. Historical novelists have to make decisions based on the (often scant) evidence that is available. Not everyone is going to agree with those decisions. If it makes the story better, then that's good.
Having said that, it's an interesting question: could Nakhtmin have been Ay's son-in-law, the husband of Ay's daughter Mutnodjmet? I see four possibilities:
- Nakhtmin was the son-in-law of Ay. He was the husband of Ay's daughter Mutnodjmet.
- Nakhtmin was the biological son of Ay. He was not married to his sister Mutnodjmet.
- Nakhtmin was both the son AND son-in-law of Ay. He married Ay's daughter, his sister Mutnodjmet.
- Nakhtmin was neither Ay's son nor his son-in-law. They were not related at all.