violet,
Think of it this way, you are invited to your husband's brother's birthday party. The house does not belong to just his wife but to both of them . You don't have to be fake but instead be warm and caring to your brother-in-law and polite to his wife out of respect to both your own husband and his brother.
Picturing positive outcomes can significantly reduce apprehension, which can lead to increased levels of confidence and an overall elevation in mood. Picture yourself having a good time at the party. Perhaps going out and buying a new outfit to wear would life your spirits.
I wish you well,
Kitt