How do I bring up a delicate wedding subject with my best friend?

Here's the short version: Becky and I have been best friends for 16 years now. Growing up, we always said we would be each others maid of honor. As time went on, that changed to her step sister. Which I'm fine with. Well, she's getting married this summer and I found out through casual conversation with her, that I'm not in her wedding. This may not seem like a big deal, but we always said we want each other in our wedding. And I asked her if she had figured out who is gonna be in her wedding party. She first said that it is 90% family and then there are a couple girls she graduated with. She's only known those girls for 3 1/2 to 4 years max.

So I want her to know how I feel but not make her feel bad and make it awkward at the wedding. I'm calling her today, how do I go about bringing that subject up and talk about it?