My previous LTR and I joked in our first years together that we'd have to start dating others in order to retain our Polyamory membership card.

My very first relationship was part of a MFM "v" relationship. That probably helped shape my own attitude about "sharing."
Because my sister was five years younger, I really didn't have much interaction with her boyfriends, just her husband, and he wasn't very sociable with family.
Since these games treat sisters as LIs, more apropos would be my girlfriends' boyfriends, in the case of pre-existing relationships, I felt it was important to be friends with them, in the case of new boyfriends, I usually encouraged their exploration. So I expect even if I was a "pervy brother," I'd still not hate my sister's boyfriends without cause.
Definitely, there's a lot of misogyny in the historic institution of marriage. People often like to point out that we are more closely related to bonobo chimpanzees than Pan. Regarding religious aspects, one reason I walked away from the RCC at 13 was the official attitude toward women ("the only way to grace for a woman was through a man" Blech).
I looked at them as a rather kinky Triple Goddess, Maiden/Mother/Crone. These other triples are pretty good as well.
You understand the -andry and -gyny suffixes!
Sauce for the goose, sauce for the gander.
I only know personally of one incestuous family. The family of a friend of a partner's friend. The kids were very close in age, and when they were young and curious, they mutually experimented. The mother decided not to stigmatize it and it's just something they did when they were younger. It did not get in the way of finding partners later in life. No apparent PTSD about it. I think because there was no power dynamics, the biggest individual danger. That's one thing NR is working hard to avoid--the father/patron is not trying to use his status to corrupt/break the girls.
I've never had incestual thoughts about my mother, or daughter (a little for a step cousin who'd been added to the family in our teens). To me, the incest plotlines are just a way to increase tension and move the story along. I've long felt that the step-incest trend in modern porn is more about how to skimp on sets and story--you don't have to explain why so much of the story takes place in a single house, or deal with getting the people to know each other. That said, LoML seems to break away from the usual fare, in that the characters actually have a back story and they aren't rushing to bed as a shortcut.
Since I've never actually had a FMF threesome with me in the center, I like to fantasize about those, but since my common experiences have been MFM with my partner in the center, I wouldn't mind them for a bit of familiar reality.