Jeb Bush's relationship with his brother has been a prickly issue for him to navigate throughout the course of this campaign so far. George W. Bush isn't as unpopular as he used to be with the general public, which means that it isn't as important for Jeb to distance himself from GWB during the general election. Moreover, Jeb is no longer the front runner by any stretch of the imagination; in fact, his campaign is pretty much on life support. Accordingly, Jeb can't afford to take the general election into consideration at this point - he's got to right the ship in the primary to avoid one of the biggest embarrassments in GOP primary history.
Jeb has tried everything else in the book, and none of it has worked. So it's perhaps not a surprise that he's now trying an open embrace of his brother. This morning's fundraising email is the first I have seen that was flat out from GWB, although I have seen several from GHWB:
Unfortunately for Jeb, while GWB doesn't inspire strong dislike in many portions of the conservative voter bloc anymore, neither does he inspire any desire to follow along with his path. In retrospect, most Republicans have concluded that Bush was a perfectly nice and decent person, and 8 years of President Obama have made us appreciate how much worse those 8 years could have been.
However, no one is particularly anxious to go back to the GWB days, where the budget was inflated with reckless abandon and principles were discarded when convenient. Further, while no one apparently has much interest in actually making someone with the last name Paul a President, their wing of the party has fundamentally changed the way the GOP views foreign policy, for better or worse.
There's no real reason for Jeb NOT to do this, at this point. There's just also no real reason to think it will work.