How can I explain this to my kids ?

I would personally explain to them its the day that your sister grew wings and went to heaven, it will be hard to explain to them but i think 7 definatley is an age where he will have a brief concept of death and what its all about. Dont worry about going into details you dont need to explain too much if its going to upset you. I think its important that you tell them the truth so they dont realize one day that you lied to them it may cause resentment on their parts. Its nothing to be ashamed of you dont need to hide the truth from them just make sure you tell them as gently and briefly as possible.

Good Luck and sorry for your loss x

At ages 11 and 7 they can understand death and they can understand being lied to. You can just tell them that September 11 is the anniversary of the death of your sister, who you loved and miss very much. They will probably be sad for you which is OK and normal. You don't have to go into all of the details of the crash as that could be too overwhelming for them.

Well, they're going to find out eventually. How would you rather have that happen, buy telling them in a way that you personally think is appropriate or by having them find out from some other source? You can't shield your kids from the world forever.

Those people who caused September 11th are hated by Muslims. They think it's totally unfair, but anyways, tell your kids it's your friend's birthday or something similar to that. Hope that helped :)

Until they're older, just say it's an "important day" and when you're sure that they're ready you can explain it to them gently :)