Some details were changed from the visual novel, either for effect or as a consequence of the truncated length. The info I got from the wiki below does a decent job of elaborating on the particulars.
The anime featured a greatly redacted story over the Visual Novel. The total runtime of the anime stands at 5 hours and 10 minutes, during which they had to squeeze in an adaption of all five Visual Novels route, compared to the average length of a single Visual Novel route of 8 hours. Due to this, there are many changes to the anime. Additionally, many additions were made to add to the drama, namely a more intense Sora hatred of Nao, that was less present in the Visual Novel.
Motoka's route is reduced to very brief Omakes that are a small fraction of her Visual Novel route.
Sora's and Nao's routes are deeply intertwined, with Sora's route in particular featuring Nao as Haru's (temporary) girlfriend; whereas in the Visual Novel the two routes are completely separated.
Sora is far less tolerant of Nao in this version.
The scene of Haru and Sora arguing, and Haru slapping Sora, is a unique scene absent on the Visual Novel.
The detail that Sora saved Haru from drowning during the suicide attempt scene is omitted. How the twins survived is not mentioned.
During the ending, the bunny appears both on the train, and ripped up in the abandoned house, which appears to be a developer oversight.
Kazuha and Akira are actually related in this version, whereas in the original Visual Novel their suspected shared blood is debunked.
Kozue is portrayed as far less tolerant and more judgemental in this version; whereas in the Visual Novel she is accepting of the twin's relationship, and remains friendly with them.
Nao running into Haru and Sora early on during the Common Route is not shown in the anime.
Sora and Haru's relationship being revealed is more dramatic in the anime. In the Visual Novel, Sora covers her nudity with a white sheet and confronts Kozue and Nao at the door, telling them to leave. In the anime, Sora and Haru are caught in the middle of intercourse by Kozue and Nao (who rudely barge into the Kasugano house).
The OST differs between the two versions. The anime is considered to have the superior OST by fans.
But yes, for the most part it's a complete adaptation. The only content that wasn't adapted in some capacity was from the Haruka na Sora fandisc sequel.