I think it depends on the wording of the invitation.
I have been invited to my brother's wedding and am single, so my invite is to 'CheekyChops and Guest' and I can take who I like. If it just said to 'CheekyChops' then I wouldn't assume I could take anyone.
We went to a wedding a few years ago where my mum and dad were invited but my brother wasn't. My dad couldn't make it so we asked the groom if it would be ok for my brother to come in place of my dad, he said that it was fine. I wouldn't 'substitute' someone without asking though, that would be a bit cheeky.