Q: Is the bride required to kneel at a wedding? What if her dress makes it difficult?
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A: There’s a long custom that bride and groom do not sit at weddings. They kneel when others sit. This is primarily because of the circumstance you cite – the bride’s dress may not be designed for sitting.
The Order of Celebrating Matrimony does not address this. It presumes that all the faithful assume the usual communal postures together.
I think it looks best when the couple sit when everyone else sits, but there is a strong custom arguing that they shouldn’t have to.