I think the girl that plays Lucy was on set, so i don’t think it is some flashback to the wedding. Also, Emma is wearing white and Ella is just wearing her regular princess dress. I don’t know what it is, but i don’t think that is it. I don’t think it is a wedding at all. Maybe they are just all together in some big ball room scene or celebrating for returning to the Enchanted Forest or just celebrating because they are all reunited.
At least we know, that there is going to be no interaction between the family and it is just going to be some sort of everyone is in the same room scene.