I have a half sister who is five years older than me. She's my father's daughter from an ex girlfriend. My father had several legal battles with her mother for years and as a result we didn't see her much until she reached adulthood. We have grown closer in recent years. When it came time to pick my wedding party I decided to keep it to five people, which are my two younger sisters, best friend since childhood, two other friends. My brothers are going to be in the wedding as groomsmen. My FH asked them on his own.
I recently found out that my half-sister is upset and feels left out of the wedding. She got quite angry with my father on the phone and said that she feels like his kids from his marriage to my mother don't consider her a part of the family. I don't really know to do without making things worse.