Carlisle, Esme, Edward, Alice and emmet have cullen as a surname. How come Jasper and Rosalie have hale as a surname instead?

Because, for the humans, to not make it wierd, they use Rosalie and Jasper as the Hale Twins. So atleast something about the Cullens make sense.

because rosalie had the surname hale before she became a vampire and wanted to keep it. at high school the story is that jasper and rosalie are twins so jasper takes the name hale as well

also- jasper and rosalie are both dating cullens.
it would be a little wierd if they were all cullen,

because they say they were adopted, & also because that way it wouldnt be weird since rosalie is married to emmett & alice is married to jasper.

Well they are really brother and sister

It's because to humans the story is that Dr.Carlisle Cullen and Esme adopted Edward, Alice and Emmet who are siblings, then adopted Rosalie and Jasper, who sagte they were twins, so it doesn't seem like insest to the people who know they're all together, as in couples. Because if they were all siblings to the human eye it would attract attention and they'd be possibly discovered.