YANBU and I don't think you're being a bitch. I think I'm like your sister and BIL in that I can be very shy, and I am definitely an introvert, but once I get to know someone I'm perfectly fine- because I've got to know them and I'm comfortable around them. I'm perfectly sociable when I'm comfortable in the situation I'm in, and can make myself be sociable, when I am not totally comfortable (e.g. night out for a friends birthday, which may include other friends of hers I don't know), but it's extremely draining. I absolutely wouldn't want to have to do that on christmas - that's why I spend it with my family 
I would hate it if someone I didn't know (and presumably had heard very little about) was invited to what is essentially a family occasion- especially christmas, as there is so much societal pressure for the day to be really special. I would feel incredibly awkward and uncomfortable the entire time I was there. And- especially if there's more than one person like that- it would create an atmosphere so the day would be a bit awkward for everyone, and nobody would properly enjoy themselves. And might the friend not feel awkward being the odd one out? Again, I would if it was me, but most others maybe wouldn't. Presumably most of the chat would centre around your family and acquaintances as well, which he would have no idea about.
Your husband definitely shouldn't have invited his friend without consulting you first, especially if there was a big discussion about your sister and BIL coming. Is it just you and them, or are there kids as well? Because christmas is hectic anyway, but it's worse if there's kids. I remember wanting to go off and play instead of sitting listening to adults talk, plus potentially more noise and mess, etc. Especially if you're doing all the work - cooking, preparing, tidying, dishes, organising drinks, plating up, welcoming guests, coats and bags, organising presents? (Also, again, if you're swapping gifts with your sister, will the friend not feel left out?)
Yes, it's your sister, but they're your DH's family too, whereas it sounds as though this friend is only a friend of your DH and not yours. Also you should feel comfortable in your own home, and you don't sound very comfortable about the situation right now, so imagine what it will be like on the day! And I think a good host should be concerned with the comfort of their guests, and this is even more important if you know someone can sometimes be a bit shy, as they're less likely to ask for another drink, where the bathroom is, etc, and just be more uncomfortable around other guests in general.
Sounds like you're going ahead with it, but if you want an out (and your DH hasn't mentioned this to his friend already), you could suggest that he's says he's sorry, but he's forgotten that it was prearranged that your family's coming so there won't be room for him on xmas day - but he can, of course, come over on boxing day.