This is all my idle speculation, not something that is necessary what I think happened. And it is more likely that all is well and there is no tension or jealousy. So don't take it too seriously.
That said, what I latch on to is this sentence from Aunt Suzanne:
It's tough always being second in your mom's heart.
And if following this thought my guess is that the open relationship worked well in the beginning when all three of them were mostly equal (and I assume that these three are the most important individuals in the harem relationship). But the dynamic changed when Mom and Dad moved in with each other and had a daughter since Suzanne would get less opportunities to be together with the other two and thus the possibility for jealousy to grow in her heart. And this leads to further speculation that she knows in her head that what they have is good and there really is no reason for jealousy. But her heart can not help but long for being the number one in Mom's heart.