The key word there is "or" - So, you can either take your immediate family (spouse, and all children under 18), or you can take two guests. Not both, so it's either your wife and kids, or your parents.

As for checking, they'll sometimes want to see the boarding pass of the people you're bringing in, but not always. That's pretty much the most checking I've ever seen. If last names don't match, I don't think they'd care too much.