A lot depends on where your sister lives and how hot it will be. I had my daughter May 31st 2012 and where we live it was too hot to use any blanket for about 4 months. We just swaddled her in thin muslin.

I knitted a cotton slub blanket, which I barely used for her. I was gifted a very simple crocheted acrylic one, which I didn't love at the time but is still on her bed now (she's almost 4!).

On a purely personal note I would have loved to receive the chevron blanket! As much as I love cotton I would also go with baby acrylics, something soft and ease to launder.