So would someone having sex with their nursemaid or orphanage caretaker also be incest?
How did you go from
things a parent does to raise their kid out of love
to a childcare professional just doing a job?
There are plenty of cases of adoptive children deeming their foster parents to be their real family rather than their biological parents with who they have no attachment.
There are also studies on the weakened parental bonding of absent parents that over-rely on hired help to take care of their children, but iirc those are mostly dependant on what stage of the kid's upbringing the help was prevalent and what are the existing cultural expectations of what a "parent" should be and do.
To answer your question, I do think a relationship with what was meant to be just a "caretaker"
could be incestual if both parties are instead seeing each other as "family", beyond the professional boundaries (which is something a professional caretaker would most certainly seek to avoid).
Not like that question is of any relevance as Simon did not sign up to be Robin's professional caretaker, he
adopted her, he
raised her, he calls her
daughter and she calls him
father. It is really hard to not see how their relationship is incestual unless you can only see the world in terms of nucleic acids.