I can relate to this problem too, although my brother and SIL just don't speak to me (her unacceptable actions towards me). My niece has just turned 17 and has more decency/integrity than both her parents.

It's your daughter's day and the realisation of why your BIL and SIL are behaving like this will go a long way to how you manage the situation. Being gracious is far more mature than cultivating their unacceptable behaviour. In other words, don't be provoked by anything you overhear. Deal with it later, if you want.

My SIL is a champion of the loud, unacceptable comment to generate a combative response. Not content with being ignored once, she then repeats hoping to get a reaction. I mastered the art of ignoring her, as she'd usually got a glass in her hand anyway!

When they realise that the event isn't about them, perhaps they'll do the decent thing and leave early.