Sadly it appears that there is very little truth to the movie. Marston's granddaughter is speaking out against the film and quite clearly stating that it has twisted the facts of their family. The poly slant seems to be more made up and real. It also appears that the filmmakers did not consult the family. :/
Lynda Carter, who has been very close with the Marston family is also speaking out against this.
https://twitter.com/RealLyndaCarter/status/919216536967106561
The family and Lynda Carter are both relatively conservative. So of course their (female) grandparents never had sex.
But that doesn't make it true. Sex between women does not result in children so it is impossible to 'prove' that it happened in a concrete way (unless there is some sort of a paper trail which has happened very, very rarely.) I'm an historian and it is extremely difficult to determine for sure if any two women had sex with each other. It was either strongly condemned so people hid their experiences or it was literally inconceivable so that two women have sex with each other may not even think of it as 'sex' since sex was only penis in vagina. For a long time, English culture didn't recognize that women could have sex with each because no bio cock was involved. (There was even a legal case where a woman accused of having sex with another woman was acquitted by the judge because in his eyes, clearly women could not have sex with each other.)
If you are wondering about male-male sex, in Western culture, that was almost always condemned and illegal - because there was penis involved, it was sex of the most perverted kind. There are hundreds of court cases involving what we would call gay sex across the West - and since gay sex was seriously feared as a major disruptive act, unlike woman on woman sex, it got a lot of attention in the medical, legal, social, and cultural arenas.
I highly recommend the book about these three people, 'The Secret History of Wonder Woman' by Jill Lepore. Lepore is one of my favorite historians and one of the best in my opinion. She does not definitely say that Olive and Elizabeth had a sexual relationship because the 'proof' is not there. They never wrote anything definitive down. All three kept their children in the dark about the nature of their relationship - Olive's children fathered by Marston were said to be fathered by an imaginary deceased husband. This was told to the children.
As skadhi notes, there is a great deal of 'circumstantial' evidence that the two women had a sexual relationship at some point, likely a romantic one as well. They lived together for the rest of their lives even after their mutual partner, Marston, died. There is no evidence I am aware of that they had other lovers after Marston's death (although if they did, this would not surprise me).
So yeah just because the family was unaware (or wanted to be ignorant) of their grandparent's relationships, doesn't mean it didn't exist.