Hi @SlyPork and welcome to the forums.
I can tell that your brother’s behaviour is distressing you, and straining the family dynamic. Although the things you have described are seemingly subtle things, you are in the best position to know your family and culture, and be able to identify that something feels wrong to you .... and you are entitled to your feelings. It was also brave of you to come and share these misgivings here ......
What can you do ? From my perspective, as a parent myself with adult children who struggle with mental health issues, this is a responsibility that lies with your parents, and with your brother himself. What you can do is speak in a safe space (somewhere with strict confidentiality codes) to a counsellor, which might be a pastoral care person at your school, or your church pastor, or your family doctor, because your concerns deserve to be heard. Especially if there is found to be a problem with your brother’s personal development or mental health, it is important that you don’t feel overlooked or without a voice. I have two siblings with mental health issues, so I understand something about how you are feeling.
Take care, and please keep writing here as you feel the need to.
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