This happened to the Vatore siblings just last night in my game and it was inexplicable because no one was culled. Lilith had my Sim's baby, then Caleb convinced my sim to leave his ghost wife and marry Caleb. Caleb was listed as the step-dad to his own niece then and I thought maybe that broke the game because suddenly he and Lilith were strangers even though she was still isted as his sister and they were all living at Wolfsbane manor: my sim, his baby with Lilith, Lilith, and his new husband Caleb.
I shrugged and just had Caleb and Lilith chat away till they were good friends again but wondered what caused it. I also downloaded a house from thr gallery and had Caleb and my sim move into it once the baby was a toddler, leaving her with her mother, and they could not wooohoo at all in the new house. I haven't tried moving them back in with Lilith and Malia to see if it was the house or the game but will have to do that next.