Is it ok to propose at someone else's wedding?

A story about a bride using part of her wedding to highlight her brother's proposal to his girlfriend is all over the wedding forums right now. According to Gulf News, it was the bride's idea to tweak her bouquet toss for her brother and future sister-in-law. When it was time to toss the bouquet, the bride asked the GF to take the bouquet as the brother walked over and got on one knee. 

​When I first read the story's headline, I thought that it was sweet, but I doubted that I would be so generous at my own wedding. This is, after all, the couple's very special, and likely painstakingly planned and expensive, day. But after reading the story, having this romantic addition towards the end of the celebration must have been really beautiful without stealing the couple's thunder.

What do you all think? Is it ever appropriate to propose to someone at someone else's wedding?