From the Major League Agreement & Rules:
Rule 20 -
(a) OWNERSHIP AND FINANCIAL INTERESTS. "No Club, or owner, stockholder, officer, director or employee (including manager or player) of a Club, shall, directly or indirectly, own stock or any other proprietary interest or have any financial interest in any other Club in its League, provided, however, that any owner or stockholder of a Major League Club (who is not also an officer, director or employee of a Club) whose interest does not exceed 5% of such Club and whose interest does not constitute a control interest [as defined] may own a non-control interest not exceeding 5% of any other Club or Clubs in its League, unless the Commissioner determines that such ownership would not be in the best interests of Baseball."
Well that's clear, isn't it?...