Appropriate or inappropriate to wear?

I was invited to dinner with the in laws to celebrate one of my sil’s birthday. The other sis had made reservations almost a full month in advance, because it is such a nice upscale restaurant. His sis made sure to emphasize to dress nicely and even sent us a pic of the dress code they have at the restaurant.

So whenever I’m told to dress up I automatically think throw on a dress and heels but I’m wondering if I should tone it down since it’s his sister’s birthday?

Plus his sister’s aren’t very girly and probably won’t be wearing dresses or heels. But I’m more of a dress and skirt type of girl, not much for blouses and flats. In fact I don’t even own a pair of nice flats 😂 😭 I only wear flip flops (mainly), sneakers, and heels.

So would it be inappropriate for me to wear a dress and heels? Or any other suggestions?