I think it's the fact that your 14 year old is so new to womanhood. I think you might need to get your 16 year old to help you with your 14 year old. It could be she's getting unwanted attention form people at school or even your son could be teasing her and making things harder. They may not even know they are doing it, little comments can be all it takes. Heck, I remember my younger brother rummaging through a suit case looking for one of his shirts but instead he yanked out one of my bras, being 8 years old he waved it around hollering my name to get me to claim it so he could keep looking for his shirt. I was mortified.
Talk to her, keep talking to her, get your older girl to talk to her. She may just be more sensitive, she needs confidence in herself in her body.