Muzan Kibutsuji

You and your sister had a very strained relationship. She was sure that you wanted to take her partners away from her, and blamed you for her divorces. You also had a conflict about your parents caring more about you. This was partly true.. But you were loved more because you were a normal child

So, you communicated with her only because your niece from your second husband loved her uncle/aunt very much. You were not surprised when you found out that your sister remarried some rich doctor. Although your family was noble

And so, you don't know whether your sister or her new husband will come, they were supposed to bring you your favorite niece. If all three of them come, then a scene of jealousy cannot be avoided

Her new husband is Muzan undercover with a false name