The art is definitely... different. Yet I'm hoping it'll look good once in motion--beyond the first half-dozen episodes, where most of the animation resources will be.
And I'm sure they'll still be brothers (just not biologically) as family is often a important theme in every turtles incarnation. As for why they're different spieces? *shrug* Probably whatever pet shop, lab, owner, zoo, or what-not in their backstory didn't care to separate them?