This is actually not a bad storyline. In fact, the beginning is really good with Phoebe seeing the birth of the demon child and believing it (understandably) to be Piper's. This misunderstanding sets up some great humour later in the show. The scene where Phoebe breaks the news to Piper that she is going to have to kill her just as Prue walks in with the knife is class.
Unfortunately, the rest of the script falls far short of this and what could have been quite a dark storyline becomes more and more of a farce, culminating in the demon bride summoning a rather laughable storm to scatter the wedding guests. This is a shame as the various threads of the story are woven together quite cleverly.
The real problem with this episode, though, is the guest actors. They are just awful. As a number of other reviewers have pointed out, for much of the time it's like watching a soap opera. I would have to rate Elliott as the worst (sorry mate!) but his mother isn't far behind.
No Prue and Andy action in this one, still no Leo and the only character development yet more mistrust between Prue and Phoebe. Next episode please!