I'm so sorry for your loss, OP. I'm in a similar boat, though not quite the same. My mom died quite suddenly in early December last year. I barely remember last Christmas, I was just in a total fog the whole time. I was on autopilot. We had existing plans to spend Christmas in England with the in-laws, and we didn't change those plans, but I just floated through the whole trip like a zombie. I ended up not coming down for Christmas lunch because I couldn't face getting out of bed on Christmas morning without my mom. DH and my in-laws did everything so the kids wouldn't be cheated out of stockings and Christmas lunch and present opening.
I'm nervous about this coming Christmas as well, because this will be the first Christmas without her that I have to cook the meal, and I'm really worried about falling apart because I can't call her in to ask her what she thinks of the consistency of the cranberry sauce. Also, this will be our last Christmas in Canada (moving to England in the New Year) and I desperately want it to be a happy memory for the kids.
I'm sorry, I'm not being very helpful, am I? It does get easier with time. When I look at myself now compared to the zombie I was 11 months ago right after she died, there's no comparison. Yeah, I'm dreading this coming Christmas, but I'm pretty sure I'll be able to function for most of it, unlike last Christmas.
Can you do a very low-key Christmas for yourself and your dad? One of the hardest things for me last year was getting off the plane and walking into my in-laws' highly decorated house, with the big tree and all the presents. It felt like a personal insult when my grief was still so raw, that they were daring to celebrate. Even though they were actually pretty thoughtful to me, I was just so damn angry. I think low-key is better than the usual splash, because it recognizes that you and your dad are still grieving.
If you don't want to cook at your place and you don't want to cook in your mum's kitchen, is it possible for you to go out to eat? I realize it costs the bomb and you have to book well in advance, but it could be money well spent. Or if you cook something very simple, a roast chicken instead of a turkey.
Good luck.