A lot of people find themselves missing loved ones moreso around the Holidays, this is a known fact. For me, it isn't Christmas, I seem to miss those who have passed on New Year's Day - I start thinking "Wow, another year gone by...so and so would have loved to see cars park themselves now", etc.
You had mentioned that your mother passed 6 years ago and why would you still be depressed. I am just throwing this out there, but death is eternal. I recently lost both my paternal and maternal grandfathers...within 3 weeks of each other. Of course I was sad and upset when they passed...and as time goes by it does get easier, BUT at the same time I am missing them more because with time I am SEEING death is forever - I am having a hard time explaining what I mean. Like this; I wake up and wow, it's March 1, 2008...and Grandpa G is still dead. I miss those who have passed more and more, rather than less and less because each day makes it more permanent.
I would think in your case, Christmas...it may just be the simple fact that you miss spending time with your mom on Christmas. Did you have things the two of you did "traditionally" on Christmas or even just getting ready for the Holidays? Things like that would easily make someone depressed.
Sorry about your daughter & son-in-law. It is always sad when people separate...only the couple knows what is best for them and you have to be a bystander hoping it all works out for the best.