Tell them "big family" does not mean it's "happy." It is not in the least bit "selfish" to ask that your NEEDS be met. (Does not apply to what you want, just what you really need.) You're going to have to explain exactly what needs are not being met, why you feel the way you do. E.g. if siblings are abusive, say so. If you have been "lost in the middle" & feel dismissed, unloved, uncared for & not cared about, say so. Whatever gripes your a$$, you're going to have to explain what's making you so unhappy, unloved, and like an outsider in your own family.
If they cannot or will not listen & try to understand, get counseling. See your school guidance counselor, psychologist, etc, to start with.