It won't happen, especially if Hillary becomes president. Then, it will be like this:
Bush (89-93)
Clinton (93-01)
Bush (01-09)
Clinton (09-13)
Bush (13-)
Now imagine if Hillary and Jeb were elected to two terms. 
The dynasties could theoretically last quite a long time, actually:
George Herbert Walker Bush (1989-1993)
William Jefferson Clinton (1993-2001)
George Walker Bush (2001-2009)
Hillary Rodham Clinton (2009-2017)
John Ellis Bush (2017-2025)
Chelsea Victoria Clinton *(married name?)* (2025-2033)
George Prescott Bush (2033-2041)
etc.
Of course, they won't. Jeb Bush can't be elected at this point, and I doubt Hillary Clinton could be elected, either.
It'd be even more frightening if Chelsea Clinton married George P. Bush. Then they would practically assume monarchial power.
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