I am so sorry for your losses - you had a lot at 10, the same ages as your younger child. And now you have 2 more losses.
First of all, don't beat yourself up with "the tree should have been up" earlier. Lots of people put theirs up this past weekend, and some haven't done it yet. So you're on track - maybe behind what you would have done in another year, but still acceptable! Your child may not remember that the tree went up in past years much earlier.
Can you ask him what he needs the most? What are his favorite parts of the holiday, and what can be left alone? Maybe you always play the music, but maybe that's not essential to him. Maybe he doesn't notice cards on display, but maybe he likes a wreath on the door. Or maybe the opposite. Does he love to make cookies? Is he happy with bakery cookies? Can you pick, say, 3 things he really wants, and then find a way to do them (even if someone else does the baking or the assembly)?
Would it help (or not?) if the 19 year old came home to help? Would your 10 year old like to do some shopping himself? Can you give him a small budget and let him step up? Maybe he really wants to buy something for you and your aunt? That might make him happy, and it might warm your heart more than you can imagine.
Would it help if you did something completely different, rather than do the same things and feel the absence of special loved ones? For example, what if you got in the car one night and went to visit some of the crazy light displays in your area? It takes no preparation other than maybe googling some of the info, but nothing needs to be bought or wrapped or dealt with in a store full of pushing, frazzled people. Have fun calculating the wattage needed, or the number of feet of extension cords, anything.
Everyone grieves differently, and you may not grieve the same was next year as you are right now. Just scale it back, and don't be afraid to talk about your loss. He's feeling it too, in some way - be sure he has a chance to share his feelings without worrying about making yours worse.
Do look for a grief support group too, as TF says. She knows.