You've kind of stumped me on this one; There's nothing holding that shirt in that configuration but the artist's willpower. Why they haven't slid off her shoulders already is an eternal mystery... cut any shirt's neckline too wide and it'll be off your shoulders before you turn around.
The only thought that comes to my virus-laden mind is that maybe you should reinforce the neckline at the sleeves (like with boning, wire, or piping) to help keep them on the shoulder.
If you can get that worked out, I think your pattern is right on the money. And Amara's link is a great one for generating the sleeves, as long as you have a thick (4"?) cuff at the bottom that is tight to the arm but not so tight you can't get your fist through it.