I need info about caring for my disabled older brother. Do I have to become his POA or legal guardian, or is being his brother enough?

You will need a Power Of Attorney for health care (a health directive) in order to make health decisions for him. You'll also need one for financial issues. It's important to get these done as soon as possible.
As far a free programs, every community is different. Since your brother is on disability, he's likely on Medicaid. There may be some subsidized programs for him. Check with social services. Also, try your state website. If you type in the name of your state and "aging services" you should get your state's version of the National Family Caregivers Support Program. The name may be slightly different, but keep looking under words such as "agency on aging" along with your state name. With some research, you should be able to find many links to aging services in your community. Try each one, mentioning your brother's status.
Best wishes,
Carol

Depending on the specific disability, there are systems in place which will step in to get the most appropriate services for your brother. For example, if has Mental Health issues contact your county MH office . There are wonderful group living situations which will evaluate him and refer to what is appropriate. The same for the folks with Mental Retardation.You may find that eventually he will blossom if the proper support is provided, even though there may be initial resistance. Do not be afraid or feel guilty for asking for help, that is why these services exist , to hwlp. if you do not , no one will benefit due to resentment.