Well, with no pictures and not knowing whether they were put on with locktite, I am guessing. So, if their is no corrosion or locktite on the threads, you should be able to get them loose with some extra leverage on your box end. I did mine with extra leverage. If locktite or corrosion is present, then a little heat might be advisable. Remove the brackets from the bike to do that.
I have a very tiny wife that can't reach them properly
My SO is short enough that the pegs are too low. For best passenger comfort, their thigh should be parallel to the ground when seated with their feet on the pegs/boards. I had to raise the passenger pegs 1 and 1/2 inch for my 5ft tall SO to be comfortable on long rides.
Bigwolf