Really though, isn't it weird that Applebloom is so young, and yet Mac and AJ seem to know very little about their parents? When AJ went to find her cutiemark, her parents weren't there. They were dead long before Applebloom was ever born.

I think Applebloom really is AJ's daughter, and she was raised as her sister because AJ was too young/irresponsible to be a mom at the time. That's why she's so overly protective of AB