These milestones usually happen on separate occasions. At least, I hope so. A wedding should be a day about no one but you and your beloved, although that’s rarely the case. The first of a child is about the immediate family, although your extended family will absolutely give you their solicited (and unsolicited) advice. For OP, it seems like her wedding might suffer from a collision of two milestones when her sister asked if she could do her baby’s gender reveal at OP’s wedding. Maybe that will fix her “something blue” issue?