My dad was diagnosed with dementia in 2012, my brother got his signature to change the trust my parents made, is it valid?

I believe that only the Attorney who drew up the Trust can make any changes, and your Dad would need to be present at the appointment.

Usually the Attorney will ask the rest of the family to leave and will talk to your Dad alone, thus this gives the Attorney a chance to see if your Dad is able to understand legal documents.

How odd.

Any other changes made? What reason does your brother say your father had for doing this?

I ask, because there could have been sound practical reasons for it (though it's hard to think why, in that case, it wasn't discussed with you). What are your main concerns about what's been going on, and about what else is happening now or might be in future?

And has your brother given you a copy of the revised trust document?

The original trust says I have 1st POA and my brother has 2nd POA. He said the 2nd trust, as he called it, names him #1 and me #2. All done without my knowledge, a couple years ago?

Has there been a formal assessment of your father's legal competence? The dementia diagnosis is a start; but if you want to challenge any change made to the trust you will need to show that your father lacked the mental capacity to understand what he was doing, and you'll need a bit more than that.

Does either of you have power of attorney for your father?