Sister Parks

It's crazy to think that when they opened the Illinois and California Great America parks were pretty much identical. It didn't take long for the IL to stand out as the cash cow and start constantly building the worlds biggest and baddest roller coasters (American Eagle, the worlds tallest and fastest coaster opened in 81, Shockwave, the worlds tallest and fastest coaster as well as having the most inversions opened in 88, Iron Wolf was the first coaster built by B&M when they became their own company [opened in 90], Batman was the worlds first B&M invert [opened in 92], Raging Bull the worlds first hyper twister as well as [tied] first B&M hyper opened in 1999 [Apollo's Chariot was built at the same time], the countries second impulse coaster and [tied] first Vekoma GIB were both built just a few years later in 01. Now SFGAm has 15 coasters including 4 woodies (five if you count Eagle as two).

Paramount seemed to go a much safer (or IMO neglected) route. They did get a B&M standup the year after Iron Wolf and a B&M Invert a year after Batman but other than that they really only pushed the bar with Stealth which was a flop that only lasted a couple years. The year SFGAm opened Shockwave, the worlds biggest and baddest coaster, Paramount committed the ultimate sin and removed Whizzer to replace it with basically nothing. In 2011 Dick Kinzel even went as far to say that they were purposely not giving it anything new because it was likely they were going to sell it to the 49ers who would close the park. Matt Ouimet killed that idea as soon as he became CEO and started building new rides and now they got Gold Striker and Railblazer.

Then of course there is Kings Island and Kings Dominion, both built Taft Broadcasting (later sold to a new company the people who were running the parks made then later sold again to Paramount then of course later sold again to Cedar Fair). Kings Island was popularized in culture by a Brady Bunch and Partridge Family episodes and Kings Dominion played a very prominent role in the movie Rollercoaster.

I don't know that I would call TDS and DAK sister parks, as they had different design teams and don't share any common layout or attractions, save Dinosaur/Indiana Jones. (Heck, I'd argue that Islands of Adventure is more of a sister park to DAK, given the defection of Imagineers from the old Beastly Kingdom land in DAK to the IOA project.) But yes, DHS/DCA has evolved into a sister park situation now, following the 2012 renovations at DCA.

Another interesting and unusual sister park candidate would be SeaWorld San Diego and Six Flags Magic Mountain, which both were designed by the same team. Obviously, they've evolved into very different directions over the years, but the path layout and central observation tower at both reveal their common roots.

SFGrAm and CaGrAM, and KD and KI are both great answers. Obviously BGT and BGW also fit, as do the SeaWorld parks. If you want to include now-closed parks, the old SeaWorld Ohio (SeaWorld's second-ever park) was later owned by Six Flags and then Cedar Fair on its way to the wrecking ball, making for another odd sister park arrangement.