ALO players
From Volume 3, Chapter 1, during the scene where Kirito first logged in into ALO:
Next I chose the nickname for my character. I didn't put much thought into it, but entered the name «Kirito».
This name was a shortened form of my real name, Kirigaya Kazuto, and there weren't many who knew that. Those who understood included only the rescue team from the Ministry of Internal Affairs, and those with a close connection, i.e. Recto's president Yuuki Shoujou and that Sugou. Of course, that also included Egil, and Asuna, who had yet to wake. Even Suguha and our parents shouldn't know of it.
In the SAO incident, none of this information had been made public, especially character names. This was because in that world there was frequent fighting between players and the result was often a horrible death in the real world. If unrestricted publication of this information was allowed, it would not be difficult to imagine a large number of lawsuits being filed.
[...]
With a bit of trepidation I realized the name was known to Sugou Nobuyuki, and because it is a rather well-known name I changed it from its romanized form to its kana form. [...]
According to the cited paragraph, since character names are not made public, the name Kirito is only known to the rescue team from the Ministry of Internal Affairs, Recto's president Yuuki Shoujou, Sugou, and fellow players in SAO.
ALO arc takes place 2 months after SAO is cleared. During this period of time, I would assume that it's unlikely for any SAO survivor to play another VRMMO when they had just been through a life and death experience.
Therefore, it would be natural for almost all players in ALO to be unaware of the identity of the person who cleared SAO and set all players free.
I guess that would clear the question of why no one in ALO ever questioned about Kirito's identity.
How about Suguha, then?
Suguha/Leafa
From Volume 3, Chapter 2, when Kirito crash-landed as Leafa/Suguha was surrounded by three Salamanders:
This voice with no tension came from a lightly dark skinned male player while standing up. His black hair stood in natural spikes, and his large eyes gave an impression of mischief. Behind him stretched dark greyish-blue wings that marked him as a member of the Spriggan race.
I guess Kirito's skin color and appearance is a great enough departure from his real life appearance that Suguha couldn't tell that Kirito is actually her onii-chan.
Apart from the appearance, there is also the fact that Kirito distanced himself from Suguha, after knowing that they are not siblings.
From Volume 4, Chapter 7:
Suguha's accusation that I had distanced myself from her because she wasn't my true younger sister was almost correct. I had searched the net for my family registry, but I had found the deletion notice, so I asked my parents about it. I was ten years-old. I began to put some distance between Suguha and myself, but there was no specific reason.
[...]
[...] By the time I was a 5th or 6th grader, I was already addicted to net games, without a side glance I moved straight forward. Finally I was imprisoned for two years, in that virtual world.
Due to Kirito keeping distance and the SAO incident, Suguha didn't have much chance to know Kirito, and there were many sides of him that she didn't know about, especially during the time Kirito was trapped in SAO. I think this contributes to the reason why Suguha/Leafa didn't realize that Kirito is actually Kazuto even after traveling together. She only caught on when Kirito blurted out Asuna's name after he failed to clear the Grand Quest for the first time.
