Is Simon Therese's cousin?

In fact I've realised that although Lette and Therese's dad are related they may not be siblings. She doesn't say what they are to each other but it still makes Simon and Therese related and their relationship incestuous.

I claim no expertise but do not believe even first cousin marriage constitutes incest in The Netherlands, nor do I think it did during the time period of the film.

"Dave, this conversation can serve no purpose anymore. Goodbye." 2001: A Space Odyssey