Should I be upset?

Today is my fraternitys formal and my fiance has known about it for the past three months. She went to visit her family this morning and turns out her sister is sick so she cant take her younger sister to the Gwen Stephani concert. She told me she might not go with me now to formal because since her sister is sick she is getting a guilt trip about "picking a boy over a sister" is that all I am? A boy? I thought I was her fiance!