Hello and welcome. My advice is probably not what you're expecting. Make this adorable quilt. Just don't complete it this week. Bring something else to the shower. If your niece knows nothing about your idea, she will not be disappointed. Your sister and nephew can keep the secret for a few more weeks. Have the quilt ready for your niece when she is receiving visitors after the baby is born. I do this when life is too full to make a quilt that is what I envision in the span of a week.