Oh Melissa I'm so sorry to hear all that!! What a rough day you've had! What is it about families at the moment?
Wow at your sister - I agree I wouldn't want something received like that either. Maybe she'll learn a lesson from getting caught this time. I feel bad for your Mum having to be there for it all!
In regards to your step dad inviting all his friends around - I would simply say no way! Especially if it's at your house and you're having to provide for everyone. It's about the family - can't he see his friends later/earlier in the day? Was he going to tell you or were they just going to rock up??
I hope everything sorts out for you! No-one needs all this so close to xmas!!