Hi, I know you're looking for the designer of the blouse that Sienna is wearing. Unfortunately, I don't exactly who the designer is but one would think it would be LV being that she looks like she's at one of their functions. On the other hand, I happened to come across a blouse with a similar design except this one has the 'pussy bow' ties instead of a straight neckline. They're from the company Chicwish and come in two or three colors and the price point is pretty good too. I'll put the link for it below.
Good luck in your search.