Hmm... this is a rather tough situation to be in. Is there a possibility that she's doing this on purpose? This being said, are you close to your mother? You don't have to explain exactly what you've seen, but you can just say that the way she acts and sits makes you feel that she's acting improperly. Embarrassing, I know... but this way, she can speak to her boyfriend about it, who may be able to his daughter. Especially if she is acting like this on purpose, it may cause some problems later down the line, in my opinion.
Other than that, you can make sure that whenever you're in the bathroom, that you put a lock of some kind on the door. If you don't have a lock, just tell your mother that she walks in while you're in there.
And have you ever tried speaking to your stepsister about this? I don't know what she would say, but just tell her that while you're in the shower, you don't want someone else to walk in and use the bathroom. It's quite possible that she doesn't know this bothers you...
And other than that, I suppose just try to avoid those kinds of situations. If she's always doing these kinds of things, then just try to avoid those specific situations. I'm not saying to never talk to your stepsister, but to at least avoid situations that cause you this kind embarrassment.
I wish I could help you more, but other than trying to avoid the situation and talking to either your mother or your stepsister, I can't see a way to make ~her~ actions stop. You can always change how you act and the like, but you can't change anything on her end if you don't mention it to someone, right? Good luck, hope it works out for you, and I wish I could have helped you out more :(.