Blackfire was the older sister in the previous continuity, so if they are twins, it's new. If she can fly like Starfire can, I'd consider it a drastic change to the character, kinda like Oracle walking again or Amanda Waller being skinny.
But yes, they may be now, but I am not entirely sure if Starfire's version of the past is true.
Interesting, always liked her character. Been avoiding Outlaws, but might pick this issue up if I see it. How's this book nowadays ?