It is possible the situation is exactly as you perceive it and this is just the case of a coworker overstepping her bounds. However I would like you to consider if there is a reason she is involved.
Many times a senior or lead engineer will be called in to fix/enhance/support a project once the original engineers have moved on (to other projects or jobs). She may be keeping her hands in the cookie jar because even though she is currently on this other project she either knows or anticipates she will eventually be put in a position where she is the one supporting the product. She may also have an understanding of the ultimate goal for that system that has not been shared with the team(not that I agree with that policy but it happens far to often in my experience).
How you should handle it depends on your role in the team.
Have a conversation with the other senior and try to understand her concerns. Ask her to make sure she is including you in any discussions about the project. There is a chance she is really just trying to help and does not realize she is creating problems. Give her the benefit of the doubt. She obviously cares about the project your are working on so where possible use her as a resource so that she does not feel cut out of the team or the end product completely.
This is a difficult situation. Some companies prefer their contractors just do the work they are hired for and some companies want your input. I generally find I have the most success staying out of the fight. Even if you report directly to your team lead your lead has a manager and unless you know who has the backing of management getting involved in the internal politics is generally a loser. You should do the tasks assigned to you, listen and be an ear for your lead if desired but do not take direct action.
Stay out of the office politics as an intern as much as you can. Keep your head down and do what you are told. Do not try and rock the boat. Remember that after you leave there are real people that will have to deal with the consequences of any action you take to change the dynamics. While it may seem dysfunctional to you, you are there to learn. Stand back and watch how other people handle it and try to figure out why(with out directly asking questions). Should you be retained after your internship and choose to get involved in the politics that is your decision but while on your internship remember you are there to learn.