Feedback to a Non-Direct

How should I approach a member of our team (non direct) who is a recent graduate that is doing a poor job relating and relating and working with our mutually shared administrative assistant? 

I am one of 4 team members who reports to a VP, all of us share an admin who does not report to any of the people she supports nor the VP.  Even though we all report to the VP, one of the team members is considered "senior" and provides oversight to the rest of the group (It is a bizarre and inefficient reporting structure, but I cannot change it I can only operate and function in the circumstance).  Despite the awkward reporting structure I have O3's with the admin and have truly developed a great relationship with the admin and she is an outstanding performer.  Unfortunately none of my team members are placing any effort into developing a relationship with the admin.  Moreover, the "senior" member of the team is mentoring the youngest member of our team who is a recent grad and neither have particularly good skills working with the admin and are rude to the admin.  During our O3's the admin has confided that she is unhappy with the team members.  The "senior" member of our team has an extremely close and influential relationship with the VP and is beyond helpless with how he relates to the admin and would not welcome any feedback from me.  Unfortunately he is actually hindering the development of the junior team member as he leads by example and mentors the junior member on how to work the admin. 

I have asked the admin if I could share her concerns with the my junior colleague.  She is not comfortable with me sharing her concerns.  I have provided some coaching to the admin on how to deal with the colleagues, her relationship has digressed to the point she has a poor attitude towards them have identified some things she can do to help the situation and she is doing a good job responding to feedback when I observe things that she does that contributes to the situation.   She is doing a good job responding to the coaching and feedback.

The situation is disrupting the team dynamics.  The admin is unhappy, and my junior colleague, who is high potential, is also being hampered in his development.  I genuinely want to help the admin and help coach my young colleague but do so in a way that is respectful, builds trust and is not a career limiting move for me by stepping on the toes of my "senior" colleague.