I really don't know what the writers were thinking when they were writing this episode. The episode itself was just wrong in some many ways, it's hard to remember it all. The fact that they made even Selma and Grandpa get together shows that they are running out of show ideas.
What I find the weirdest thing about this is that it's a marriage within the family; Selma is Homer's sister in law and Grandpa is Marge's father in law, what the hell would Homer and Marge be? The only thing I liked about the main story was when Grandpa was struggling with the "new technology."
The second plot was just a weak one, it was like they couldn't think of anything for Bart and Lisa except set up a storyline just so they can recreate the scene from LOTR. Not saying that the fight scene wasn't good, I especially like when the dragon came flying down because it was so unexpected.
Overall, I give this episode a 3 out of 10 for a stupid storyline and a weak sub story.