Hello Casey! One thing I have learned is it's easier to make reservations for anyone who might be dining with you and downgrade the number if needed rather than the other way around. In your case, I would make your reservations for 8, just in case your brother's family decides to go. As the date gets closer, it will be easier to remove them if they decide not go vs. add them in.
And in case you didn't know, you can make reservations 180 days prior to your check-in. You are going at a very popular time of year so you may want to consider making those reservations as close to your 180 day mark as possible.
I'm not sure if you have anything in mind for Christmas Eve but what I did with my family last week for Christmas Eve was book a Candlelight Processional Dinner package. There are several restaurants you can choose to dine at (we chose Garden Grill so that we could include Chip & Dale in our plans) and you are provided with reserved seating for the
Candlelight Processional followed by a reserved area for Illuminations viewing. It was truly a peaceful and magical evening.
I hope this helps! Diane :)