This is a difficult spot for you to be in. You love and care for your brother , but this need not be at the cost of your happiness.
Like you have mentioned, about his treatment, more details would help to plan an intervention for him.
In my opinion this situation can be worked around with counselling for you and Psychotherapy for your brother.
Next Steps
Find a mental health counsellor for you that can help you step by step of to work around this situation.
Also, your brother needs psychotherapy as he is experiencing intense separation anxiety as this girl may go away from him. He can be helped with emotion management and regulation, which will require regular sessions.
Health Tips
1. Practice mindfulness while have conversation with him.
2. Do not leave him unattended for a long period of time if you anticipate suicidal thoughts or tendency.
3. Keep suicidal helpline numbers accessible or on his emergency numbers.