Re: Bridal Shower!!
Congrats on your upcoming wedding! Too bad your mom ruined the surprise for you hehe, hope you are a good actress :CareBear:
For what to wear --I think it depends on how your sister was planning on getting you to come to the surprise shower. If for example, she had told you that you guys were going out to a dinner or party at a friend’s, then you can dress up a little, because you normally would have in this case. But if she just told you something like she wants to meet up for coffee or just go shopping or something, she might be a little suspicious if you show up in a prom dress and an updo! 
LOL
So depends on the situation. If you think they wont be surprised to see you dressed up, then chose something that you like, look great in, feel comfortable in. A cute knee-length dress maybe, or a skirt and shirt, or nice pants and a top
Have fun!