R.,
So sorry that you have to go through this with your dad being sick! I would have daughters boyfriend there regardless of what your sister thinks. If your father is alright with it then you shouldn't give it a second thought. This is more for him anyway.
Perhaps a recent event in my family can offer a little comfort to you. Recently my brother and his wife (whome no one likes) moved into my father's home (money problems). It wasn't long before my sister-in-law caused a huge scene in front of about 100 family members and close friends. At my daughter's first birthday party. (we had it at his home because we live in KY and they all live in MI). Well...my dad being the kind of person he is kicked her out. My brother and niece went with her of course. However...the whole family started saying that they don't ever want her around and that she's never welcome into anyone's home. My dad especially.
Well guess what...They all just sat down and had a nice dinner last night.
My point here is that my family loves my brother enough to respect the fact that this is his wife no matter how many problems they may have with her. And that despite the stupid things they said...she's still hangin' around.
Your sister will more than likely come around. If she loves your father then she'll respect that this may be his last Christmas and she'll just have to get over or at least put aside whatever issues she has with your daughter dating and having the baby of someone of a different race. If she doesn't, and this does end up being his last Christmas she'll feel guilty for the rest of her life.
Maybe you should sit with your sister and talk about it with her. Explain that he's a big part of your daughter's life and that you want to make this last Christmas a happy one for your dad. If your dad wants him there it's obviously because he thinks of him as family. Maybe your parents can be there to talk to her too.
I really hope all of this works out for you and your family. And congrats on the grandbaby!