I think you already know why you are guarded about expressing your own feelings. You wrote a thoughtful and insightful letter.
You had lots of experience when you were little that expressing your own feelings did little good. Your tender, normal little girl sensitivities took a back seat to the more flamboyant “freak outs” of your brother. If that weren’t enough, your parents had their own troubles and were working a good deal of the time. Where was there any room for you?
On top of that, my guess is that you knew that your family was already handling as much as everyone could. You didn’t want to add to their distress.
What started as a functional coping skill (keeping yourself to yourself) has become a habit of not sharing your innermost self. Even though there may be more room for you now that your brother’s illness has settled down some, you are not at all accustomed to openly sharing. In some sense, your “depression” may, in fact, be a learned coping skill. You bottle yourself up to keep your feelings to yourself.
I encourage you to ask for some therapy. If you could simply uncork the bottled up feelings, you would have done so already. It’s likely that you need some help learning how to do it now.
You live in a city where there are good services for teens. Just do an internet search to find a counseling center near you that specializes in teen issues. I hope your parents will be supportive. Just because your distress is quieter doesn’t mean that it isn’t as important as your brother’s.
I wish you well.