What can I do if my father told me he will subpoena my brother and I as witnesses in my parent’s divorce?

If a witness has a legitimate basis for opposing a subpoena, then they can file a "motion to quash" to the subpoena.  However, "just not wanting to testify" won't usually qualify as a way of getting out of it.  So, you're stuck with complying with the subpoena if you are served.  If you are never served, then you are not required to appear.

If you do appear to testify, your only job is to tell the truth-- regardless of which side it helps.  Telling anything less than you know and "how it is", will only cause more problems for you.  If they ask you about the gun collection, you need to tell about the gun collection.  Often, parties will say "there is nothing that can be done," when there actually is.  If you feel that it's important to help your mother, you may want to contact her attorney when you receive the subpoena and tell the attorney what you know so that they will know what relevant questions to ask you.  If you don't tell them, they probably won't explore that line of questioning because most attorneys don't ask questions they don't already know the answer to.

If the attorney for your dad contacts you, you may want to set the record strait with him as well.... he will not be inclined to call you if he considers you a adverse witness.

How involved you get or do not get is entirely up to you.

Regardless of who is asking the questions, here are some general tips about testifying.  First, your brain is likely to freeze when you hit the stand-- take a few breathes and compose yourself.  Second, look around the room, find a friendly face, and talk to that person.  In divorce cases, often the only half-way friendly face is the court-reporter-- continue to glance at them.  Third, never answer a question you don't understand.  If you tell the attorney you don't understand the question, they will be required to rephrase it until you do.  Fourth, if you don't remember, don't guess.  You're under oath to tell the truth and your job is just to tell when you know and what you don't know.  Guessing always leads to a witness being cornered by their own testimony.  Fifth, if you don't hear the question-- then don't answer the question.  Witness often "zone out" during testimony.  They will see the lips of the attorney moving... and then a "blah blah blah....", but nothing concrete.  If you didn't hear it, just ask them to repeat it. These are just general tips, but they will get you through a majority of the issues that arise whenever any witness is called to testify.