First I would like to say I am sorry for your loss. I know how hard it is to lose a Mom to Lung Cancer as my Mom passed away almost 5 years ago from Lung Cancer.
What I do to remember her on the anniversary of her death is to wear a piece of her jewelry. My Dad gave me all of her jewelry when she passed and most of the time I don't wear it because I am afraid it will trigger some emotions and I don't want to have a breakdown when I am at work or running errands. On her Birthday and the anniversary of her passing I will wear it, and sometimes I will wear some clothes of hers that I also have, like a sweater. I always get a compliment on the jewelry and my heart smiles every time.
As for my Dad - we don't talk about it much, but we do have breakfast every Saturday and I always make a point to wear the necklace on that day too - often he just looks at it and says something about it being a nice necklace. We don't really talk about it cause that is how we are, but we both know we are thinking and missing her.
Maybe you, your sister and Dad can get together and do something that your Mom would have liked? I have thought of planting a tree in her honor, even though I haven't actually done it yet.
Whatever you do will be right, as long as it is from the heart :-)