Sorry I don't know the background to your story, but I would like to send my support to you regardless. Express your thoughts as much as you want, in doing so you will be on the way to coping better.
My own dear dad died suddenly, two years ago today (5 weeks before our youngest was born). We still think about him and miss him terribly and like you, feel so sad for mum being on her own. However, this said, things do 'move on'.
We still talk about dad and often share memories with mum and our own children by way of stories, photos etc. I often find myself saying to my mum/hubby/children 'Dad/Papa would have loved ...(this place or that situation)". Not in a morbid way, but as a part of remembering him, as we often then recall a story or happening about him and it brings a smile to our faces.
In truth, I cannot say that the 'empty' or 'strange' feelings ever really go, but they do mellow with time (hard as this may be to accept at the moment).
Keep your special love for your dad a living part of your life and you will be able to move on. Share this love and memories with your mum and give her as much support as you can in her life too, after all, she has to move on also. We may not 'like' doing something that has 'sad' memory associations for us (going to the house etc) but doing these are all part of nature's healing ways.
Take care.
butterfly