Don't fret, everybody has a bad day now and again. I think that your problem is probably your 1/4 inch seam, check it and adjust accordingly. Don't feel bad, this is the most common mistake in quilting. Keep your chin up, your sister-in-law will never notice a mistake. Every quilt I have given(even by beginning ones) has been received with great joy and acceptance. She'll love it. So keep going.