I think that Didyme and Marcus, I can imagine them a lot like Bella and Edward. Like so in love and so connected that if one of them would die they would be just devastated. I think that's what happened to Marcus; when Didyme died, he just lost interest in everything in the world.
Sulpicia and Aro: I don't know. Aro doesn't seem like that loving type, you know? Maybe he is a creeper

Athenodora and Caius: I think their relationship is more like Alice and Jasper, not like super like it, but more like their privacy. Like they do love each other but they don't show it in public. I think that Caius is also like a "workaholic" in a way, haha, but at the end of the day, he does love Athenodora.
And then it's like how the wives get along. I think with Didyme there, it was a lot more happier but now, with Athenodora and Sulpicia, maybe they like each other but not a lot? Maybe just in the way, you know, "She's my sister-in-law and I care for her but not care her."