I found nursing in a stretch wrap (like a moby to be the easiest thing in the world and ir basically acted as a cover. You could try it. I did not have kids who would nurse under anything.
But to be perfectly honest if I were in that situation I would just avoid my inlaws. If they think their completely irrational feelings are more important than my baby's health and happiness then they don't need to be around my baby. Seriously. I would also by whacking my DH upside the head.