((((((((((E)))))))))), I don't know what it's like to lose a sister, but I have lost grandparents and I can completely understand the feeling of not wanting the person to be forgotten.
It's great that you, family, and friends were able to have a big birthday bash for her last year. I can tell how much it meant to you. Just because you can't do something similar this year, does not mean they have forgotten her.
There are lots of little ways to keep her memory alive that don't involve a party, and might get more participation from out of state people, too. You could volunteer to speak to high schools and colleges about the dangers of drinking and driving, you could phone in a dedication to the radio, the computer wallpaper you had was a great idea, you could ask people to make a donation to a charity in her name...
Like chi said, and I'm sure you know, everyone deals with tragedy differently. I cannot even begin to fathom how your mom must be feeling to have lost a daughter so young. Even if she does just sit on her laptop all day, I'm sure she'll be thinking of your sister the whole time and I don't think you should make her feel guilty about it.
I hope you're able to reach a compromise and find a way for people to honor your sister's memory in whichever way is best for them.