It pains me to say this but at some point in every organizational change initiative, you need to call upon the firstborns in the organization to step up and do what's responsible. It pains me because I am a lastborn and it was tough living up to the standards my oldest sister set. But it's undeniable that firstborns know how to take care of things and want to do so. It's their nature to be responsible. If I don't do it, it won't get done. If you are leading change in your organization, use it to your advantage.
You do so by first avoiding the "l" word -- leadership. What I've learned is that there are many preconceived notions of what a leader is or isn't, or what leadership is and isn't. Particularly if the firstborn is not a manager, it unnecessarily complicates matters to label big brother or big sister's behavior in the workplace -- takes responsibility for results; pulls people together to get things done; initiates difficult conversations when an issue is being ignored -- as leadership. My experience is that firstborns don't seek the title; they would consider it presumptuous of them to call themselves leaders. They want to contribute to the enterprise so put them to good use.
What I've learned the hard way is that the greatest untapped asset in an organization is a firstborn who is not a manager. He or she wants to make a contribution but doesn't want to step on anyone's toes. They respect positional authority and boundaries. But they struggle with how things are going, particularly when they see where and how they can help to make things better. My advice is to find situations where they can step in and serve as a role model. This way you avoid the confusing situation of "why are you, my peer, telling me what I should or shouldn't be doing?" Recruit them ahead of time to be prepared to share their perspective or opinion in a group setting that will set the right tone of learning, self-reflection, or embracing change. They're likely to have as much, or even more, credibility with their peers than you do. Put them to good use so that they can make the difference they want to make.