Recently an inappropriate comment by a staff member was made at my IEP meeting. Admin was present and heard it, and it was also recorded. I also emailed admin to let them know I wasn’t okay with it. Admin set up a meeting to clarify what was said and staff member wanted to apologize. One day before the meeting I was informed that staff member was bringing a union rep. I declined to meet in this way as I believe it to be inappropriate to have a union rep present to discuss a matter that occurred during my child’s  IEP.  Can union reps even be allowed do this? Isn’t this a FERPA issue?

It sounds like the meeting isn't actually an IEP meeting, so I don't think FERPA applies. If administration is present at this meeting, I could see why a teacher would want a union rep present. You can always end the meeting if your child's information is brought up, but unless your state laws limit union power I don't think there's much you can do about the rep other than refuse to attend the meeting.

It's your right to decline the meeting, but my understanding is that  it's not uncommon.

Remember, it's not about you or your child, it's about the teacher seeking their own legal protection in regard to their administration.