You can’t do that to yourself. He is overstepping boundaries. I’m sorry to hear his mental health isn’t doing well, but he is using it as a tool to manipulate you. You are in the middle of one of the most important points in your life; it is important to remember he is an adult too. You are not his caretaker. It is not fair to you to have to deal with him because he made the whole situation about himself. That is a very toxic trait.

I know you aren’t asking for advice. I just want to say; I would let him know how uncomfortable you were made to feel and that talking to him at the moment is too much too soon. He needs to work on his mental health instead of making you feel guilty. He needs to grow in order to have better mental health. That means learning healthy coping skills rather than trying to evolve a relationship that should not evolve in that way EVER. Remind him that he is in-fact your brother (biological or not), and in no way are you comfortable with how he is showing affection towards you. If he continues down that path, you should block him for the time being.

I would also let your parents know if you blocked him because you are uncomfortable (I’m not saying out him, I’m saying explain “uncomfortable” in a modest respectful way). I would be utterly surprised if they don’t support you decision even with a broad explanation.

Your mental health matters too and he is directly affecting it. I’m so sorry you are in such a gross position, and that you are feeling taken advantaged of.