Is that one buy for 2 outfits, for the little sister and big sister?
No. If you want both Big and Little sister, you have to order each one individually.
Does this outfit run true to size or should I order a larger one?
Yes, the size is as expected, fit perfect for my 2 daughters
Is it soft fabric for baby?
Yes, its very soft, breathable, and stretchy! very comfortable to wear, perfect for summer!