It depends on the venue. From my experience, though, as long as you have a ticket for that section it should not be too hard to sneak down to the front. At a concert last month I was supposed to be sitting in the fifth or sixth row but I spotted a spare seat next to my friends and claimed it for my own. No one turned up to sit there so it must have been vacant. It is a good idea to gauge the situation and if there is a spare seat it would be easier for your stepmother to put her belongings in it. It makes it look more like she belongs down there.
Just go for it though. The most they can do is send her back to her rightful seat.
Laura.