Go to the reunion -- really...
Yes, dance recitals, even at 5.5 yo, are important (I have 3 dancers), but...
I also have a son, who absolutely wanted nothing to do with sitting through his sisters' (and every other kid at the studio) seemingly endless (to him) recital. So he stayed home or spent a day out with dad while I managed the girls.
You have more than one child. Sometimes you have to divide and conquer. Not every kid or family member can, or should have to, attend every event. It doesn't mean you love them less, it means that everyone in the family is deserving of respect for their choices.
Send dad with flowers, buy the DVD, and plan a family evening to watch the performance. Sometimes, it's even better because there are close ups, and you can replay your SD's part over and over again.
Good luck to you -- although I have read enough to understand that there are more issues at play here, I think you really need to give yourself a break from the guilt over the recital. You are just beginning the process of living as a multi-child family. It only gets busier!
(lucky mom to Amelia 20, Camille 17, Evan 15, and Gracie 12)