I don't find it odd at all. Sure he existed, in the womb, for 7 1/2 months, but no one "knew him." Rebecca, and Jack to a lesser extent, were aware of him, and they both grieved shortly after the births. As far as we know none of the triplets were identical, so that bond wasn't there. I can imagine a short conversation when the Big 3 were younger about the loss, but I don't think it is a major plot point.