How to dress to impress my boyfriends family?

We're going on a picnic tomorrow, and while I've met his parents and brothers already and know that they like me, I'm meeting his sister for the first time. She's a missonairy and is just getting back from Africa. since she's his only sister I would really like to impress her and be able to build a relationship with her. She's very girl-next-door type look and while my look is mostly yoga pants centric (I'm a proffesional ballerina) I'd like to go for something similar meeting her. Any tips or pictures of outfits I could try to model? Thanks so much :)