Actually, they were definitely still alive when Applejack got her cutie mark. Apple Bloom was nowhere to be seen in Where the Apple Lies and Diamond Tiara's parents weren't married yet, so it's highly likely that those two ponies of the same age weren't born yet. Big Mac and AJ were teenagers in that episode, so they got to spend lots of time with their parents before Apple Bloom's birth.



Although it is true that Bright Mac and Pear Butter didn't actually get to witness the moment when Applejack got her cutie mark. They must have been busy elsewhere that day.