Thanks for all the suggestions! I'll try not to be too frazzled and have already written down everything that I can do the day before to keep cooking and prep to a minimum. DH thinks I'm insane - it's part of a laminated Christmas guide that I made with recipes, table settings etc. in. Is that really bad? 
I'm not expecting SIL to do anything except go to bed. I'll make a plate up for her and keep it ready for when she wakes up but I'm not expecting to see her much before everyone else goes home. I'll go with the stocking with some nice overnight toiletries and chocolate and make her a special breakfast for when she gets here in the morning.
Christmas music - yes
Nibbles and drinks - yes. What sort of nibbles do you think? I don't want everyone full before lunch!
Getting drunk - no, I don't drink :o Will have to rely on hot chocolate.
Games - yes. I'll get charades and Cranium out.
People helping - serving drinks and keeping toddler DS busy, yes, everything else probably not as our kitchen is quite small and I would probably drop the gravy.
Small gifts - we were planning on doing presents after lunch; is that ok? I thought it would be better that way round rather than when I'm busy cooking. DS is only 1 so won't know he's waiting for presents and everyone else is 18+.