Can I legally be forced to testify against my brother?

There is no privilege against testifying against a non-spouse family member, therefore you would need to testify if subpoenaed. A subpoena is a form of a court order, therefore if you fail to comply with it, you can be held in Contempt of Court. Contact legal counsel to ascertain those things that you can legally refuse to testify to.

As unfair as this predicament puts you in, you will be required to testify as long as you are a fact witness to what your brother said, provided the judge believes the testimony is relevant. (Generally character testimony is inadmissible unless the issue of character was first introduced by your brother.) There is no "brother" privilege in the state of Ohio, although maybe there should be. However, you have rights too. You can file a motion to quash the subpoena with the court, and argue that your testimony is irrelevant. If you simply don't bother to appear however, without filing a motion to quash or doing anything, you are subject to being held in contempt. If this is any saving grace, you are entitled to a witness fee, paid in advance. I would have to look at your subpoena first, but depending on whether the witness fee was included with the subpoena, you may be able to get out of it that way, but like I said I would need to look at your subpoena first.