I have one of those too, even still at 16m. It can be inconvenient at times but I honestly can't blame her. I'm a SAHM and DH works away two weeks of every month so it's her and me alone quite a lot of the time. It's not her fault I'm the only person in her life who is constant. Anyway, the point is, I know how you feel and I know I would feel the same. Quite honestly, I would probably not go for a long period if I thought she would get stressed about it. She's ok for an hour or two with my mum and DH but, boy, if I wasn't there for routine times like bedtime, it would upset her a LOT and I'm just not prepared to do that to her while she's not old enough to understand that I will come back.
That's just my take on it and I know a lot of people would believe you shouldn't let your child dictate your life but I figure I chose to have her so I am prepared to sacrifice things for now.
I'm not saying at all that you shouldn't go, but just letting you know it's ok to do that if it is really worrying you. Don't worry about what people think.
Another thought - could you perhaps take her to the ceremony, and drop her with someone afterwards so it wasn't such a long time apart? Or even work your routine that day so that you could put her to bed early then leave for the reception?
I do feel for you and hope you're content with whatever you decide to do.