tough one…very complex relationship. This dynamic could be related to dominance and passivity in the personalities of you and your brothers. Maybe, when they protect you, you send them subtle messages that you like it. Maybe they believe you are vulnerable to harm and their love for you won’t let you fall victim. Maybe your sister is tough and knows how to take care of herself, and has proven to them she can “hold her own.” Maybe none of this is true, but just because you have always been the youngest they continue to perceive you as “the baby,” even though it may not justified. Motivations for our attitudes and behaviors in relationships are SO complex, and invisible, you (and they) may never, ever really know why they treat you this way.