A family member can go to court and seek guardianship if his safety is at issue. Hire a local lawyer to do so or got directly to the local probate court and talk to the clerk.
You have a very difficult situation. In Maryalnd, there are only 2 ways to manage a person or his affairs - through a power of attorney or a guardianship. He won't sign the necessary POA. In pointing that out you are saying essentially that he is legally competent - he couldn't sign a valid POA if he were not legally competent. So the only possibility is a guardianship. They come in two forms - Guardianship of the Person and Guardianship of the Property. In my experience judges scrutinize requests for Guardianship of the Person far more closely than Guardianships of the Property. The problem for you is that it is likely your sibling will fight a guardianship. Contested guardiansghips can get very expensive (as to legal fees) and time consuming. Add to that you will be filing a legal proceeding against a resident of your household. That will likely bring on a whole other set of problems.
In order to manage his medical care, you will need a Guardianship of the Person. In the area of psychiatric care for someone who is legally competent, the problem is that he can consent one day and withdraw the consent the next.
Finally, you likely have issues that are not just legal - is it appropriate that you should take on this responsibility? Is it legitimate for him to take from you - a place to live - but refuse your help? Are you possibly enabling his behaviour? I have no idea whether any of these might be applicable but if they are some kind of joint counseling might get your sibling to the point where he becomes agreeable to allowing you to take control.