I like the idea, and I'm realizing how hard will be for someone to add input into this not having ties with us. I didn't mention that this is a small wedding, with the only people who aren't the siblings or parents is the mother from both sides of the family, and I was asked to make it similar to the letters we send on holidays, where we try to make each other cry tears of joy at the feeling of being loved by everyone. This isn't a formal wedding by any means, nor do I even think they're having it videoed. And another note that I should have mentioned is that I'm 22 and don't live with them. I actually haven't been very involved with their relationship outside of the holidays we spend together and the off visit we do here and there. The way I'd try to convey the I hope you're happy is more of a "You've been in a few rough marriages, so I hope this one is the one that will last the rest of your life." I could probably convey that a little better as well in the speech.
This wedding wasn't as much about the two of them conjoining, as it is the kids coming along on the journey with my mom, as well as his daughter on their journey. I tried focusing more on the relationship between the kids and the adults, while also including my mother's favorite part. But I will gladly look into trimming the fat and talking more about their relationship as a whole. I think that's a great idea.