If you are the younger brother or sister of an older sibling, you don't get invited because you aren't wanted. And you aren't wanted because you are annoying (on a good day).
One of the problems of being the younger brother or sister is getting what you asked for. You spend all day pestering your older sibling (and pestering, and pestering, and pestering, and...), because they are doing something that you like, and you want to do it too.
If it actually works and you get to go, it's grudgingly. Sure you're there, but you don't get to enjoy it, because no one wants you there.
The Old Testament reading makes it clear that God's invitation is not like that at all. You are invited because you are wanted. And you are wanted because you are accepted (even if you are an annoying younger brother or sister).
More on Sunday's readings tomorrow.