Invitation question: I'm inviting all my co-workers and their spouses in my small office. I have become really close to one co-worker's granddaughter. She is 13 and part of a youth group that I help out with. The granddaughter and her mother live with my co-worker and her husband. I have never met the granddaughter's mother - from what my co-worker said, she lives at their home but granddaughter is basically raised by them. My question is, I am inviting coworker and her husband, can I also invite the granddaughter but not invite her mother? I literally have never laid eyes on her mother and think it would be quite odd to do so, but I don't want to breach etiquette by inviting her child and not her.
Thoughts?