Sizing/fit for the blouse

Hi, all - I've mostly sewn for my toddler son and, as a side effect, my toddler nephew, but I'd really like to make a couple of gifts for my older nieces before seeing them this summer. I've loved this blouse pattern since it was released, so this is a nice excuse to sew a pattern I otherwise don't have a reason to own. They're not local, so getting measurements is probably not going to happen, but I do have their sizes. I'll probably size up for my younger niece, but my older niece is a size 12 or 14, according to my sister. She's very tall and very skinny. How forgiving is the fit on this blouse? If I make a size 12, could I add a couple of inches, or is that asking for trouble without measurements?