Mia's older sister is "sixteen and in the process of wearing down our parents' patience. She changes her hair color the way normal people change their underwear" (1.6). That's one way of saying Beth's fickle. She goes through a lot of trends—and hair colors—in the book. She's at that stage of the teen years where she's testing the waters of different ideas before figuring out who she's going to be.
Beth comes back from camp having decided to become a vegetarian and ready to lecture the rest of the family on the ethics of eating meat. It's not long before she hunts for her own herbs in the woods, too, only to come down with a bout of poison ivy. While we're all about following your interests, Mia makes a point about telling us that it's not so much that Beth believes in this stuff as it is that she goes along with an idea for a while before changing her beliefs again next month. Beth, it seems, is always changing.
But hey, isn't figuring out who you are what being a teenager is all about? Mia's also trying to figure out who she is, changing friend groups and experimenting with things like acupuncture to figure out who the heck she wants to be. Beth just happens to do her exploring in the woods.