I am LOLling like a mad woman here - imagining lovely pic of bride and groom gazing into each others' eyes, and SIL in the background, one breast on full display, with her mouth open shouting 'breast is best'. Or when they are cutting the cake, there she is again, the other breast out now, while she waves a banner with a bf slogan on it. And whoops....there she is again, half-nekkid, as they sign the registry.
As Pastabee says, if it happened like this, I can perfectly accept the bride is resentful.