Do I have to appear in family court under a subpoena to testify against a sibling of mine under a child custody case

This is somewhat brief of a question, but we can, for this response, conclude that a) the Subpeona was properly issued and served, b) there is an ongoing custody matter for trial on that date of the Subpeona, and c) you have no attorney. The third matter is key.  You need someone to review the documents and go with you to Court.  It does appear that you ARE being compelled to testify.   A litigant has the right to compell the attendance of any witness.  Any failure to answer can lead to contempt of Court.  As such, any Subpeona is not to be taken lightly.  An attorney may be able to limit these issues &/or have any "testimony" be in Chambers (as opposed to being in open court).  The attorney can also insure that you are not incrimated with your testimony.  In pre-court discussions with the representing attorneys, your testimony may be either limited or avoided. As such, the general answer to your question [with out seeing the document] is YES, but the recomendation is for you to, at least, consult with a local attorney experienced in these matters.  The documents can be seen and reviewed. You can then also be informed of the process and procedure in these courts even if the attorney is not retained.  Your local county bar association should have a referral system, whereby they can give you some names of experienced attorneys.  You can then view the various websites and make a knowing choice for setting an interview.