It can take some people longer to feel comfortable or even to be able to really deal with the reality of associating a very close loved one with a grave. I was 21 when my father died, and it took me a long time to be able to go to the grave.
I've found that losing someone very close seems to take about five years to really get over (as much as one can ever get over it). Something that backed up that belief was when Diane Sawyer covered the September 11 surviving family members around the fifth anniversary of the event. She noted that the families seemed to have reached a different stage and had begun to move on, compared to earlier years. That's not saying someone isn't "better" at three years than at one year, but it's a lot slower a process than a lot of people often realize.
My husband's sister and her husband lost their 20-month old child, and the husband couldn't go to the grave.
Also based on my own experience, I found losing my father at 21 a much different thing than losing my mother when I was 41.
I'm guessing you may be able to deal with it better after some more time passes. (My sister and I go our parents' grave regularly, and we're really used to it.) If you're young that could be a factor too. I, personally, don't think it's either "weird" or "
When I couldn't go to my father's grave I used to tell myself that he would have understood why. I'm now the parent of three grown children, and I'd want them to know that I, too, would understand if it was something they couldn't do. You've got the rest of your life to go to their grave sites. You probably need more time; and if you do, your parents would probably understand and want you take the time you need.
Siblings can be very judgmental when another sibling doesn't think and act the way they do when it comes to something like this. No matter how close they are, siblings "walk in different shoes" when it comes to their individual relationship with parents or when it comes to something like differences in age/emotional maturity if parents pass away. Each person must deal with grief in his own way, and your siblings need to understand that.
For you to lose both parents so close together is an awful lot to deal with for anyone, whether you're young or old.