*sigh* That's a tough question. I love my older sister so much. It would hurt to know that for who knows how long, I'd been talking to someone else. Knowing she was in there the entire time. What's even creepier, is if she told me about this "great new group" called "The Sharing" I would totally go with her and join. Even request to be a full member. Amazing, how people can have such a big influence on younger siblings.
If I knew? Well, I'd panic, at first. I'd probably grab my money stash, pack a bag full of food and water, and head out the door. I'd probably come to my senses, though, before my absence was noticed, and return home.
Perhaps I would make an attempt to starve the Yeerk, but that would be difficult, considering I'd be fighting a nineteen year-old, as well as a very angry alien who would not hesitate to hurt me. I'd have to find a place to keep her for three days. Yeah, I don't think I'd be able to starve the Yeerk.
Like morf, I'd have to just deal with it and pretend I didn't know. Getting killed or infested wouldn't be worth it, considering my chances of actually freeing my sister would be unbelievably small. And if I were infested, I'd have to worry about freeing both of us. Couldn't handle that.
If I were an Animorph, now, I think I'd be tempted to do something very bad to that Yeerk. And I WOULD, more or less, have a way of keeping her hidden and tied up for three days...
If I ever found out she was voluntary... I don't know what I would do then.