I don't get what's so hard to understand about their relationship. It looks like it's a polyamorous one where not everyone is in a relationship with everyone. Lol. It's pretty simple and I'm pretty sure this was a common thing in the past, just not as harmonious. Read to read people, not to cast judgement on specific ideas
At this point, i feel like this is a drama between the wife, husband, and his mistress, and how the mistress is getting approval from the wife. Meh ://