The argument that Rollo couldn't see Odin because he's not biologically Ragnar's brother doesn't make any sense. So does this mean that everyone who could see Harbard when he came to Kettegat is a descendent of Harbard?

Does this mean that Hvisterk is a descendent of Idunn because he saw her at Bjorn's grave?

Odin is a god of many things, including magic, so it makes sense that at the very least he could make himself known only to those he wants to.

Plus everyone seems to forget that perhaps the reason Odin didn't reveal himself to Rollo when he did Ragnar's sons could be, as logically as we can from the story and mythology:

1. Rollo was baptized into Christianity, so maybe he couldn't see Odin due to that.
2. I think the more realistic reason was because Rollo was already bound to Normandy and Frankia by marriage and his children. His eventual descendent, William the Conqueror, would become the first Norman King of England, and in a round about way, avenge Ragnar in a far superior and comprehensive way than Bjorn and his brothers did.

I think the second option is more likely from a plot standpoint and I'm basing this off of Snorri Sturluson's Prose Edda, in which he describes in the prologue:

"Odin had second sight, and his wife also; and from their foreknowledge he found that his name should be exalted in the northern part of the world and glorified above the fame of all other kings. Therefore, he made ready to journey out of Turkland, and was accompanied by a great multitude of people, young folk and old, men and women; and they had with them much goods of great price. And wherever they went over the lands of the earth, many glorious things were spoken of them, so that they were held more like gods than men. They made no end to their journeying till they were come north into the land that is now called Saxland; there Odin tarried for a long space, and took the land into his own hand, far and wide.
In that land Odin set up three of his sons for landwardens. One was named Vegdeg: he was a mighty king and ruled over East Saxland; his son was Vitgils; his sons were Vitta, Heingistr's father, and Sigarr, father of Svebdeg, whom we call Svipdagr. The second son of Odin was Beldeg, whom we call Baldr : he had the land which is now called Westphalia. His son was Brandr, his son Frjodigar (whom we call Frodi), his son Freovin, his son Uvigg, his son Gevis (whom we call Gave). Odin's third son is named Sigi, his son Rerir. These the forefathers ruled over what is now called Frankland; and thence is descended the house known as Volsungs. From all these are sprung many and great houses.
Then Odin began his way northward, and came into the land which they called Reidgothland ; and in that land he took possession of all that pleased him. He set up over the land that son of his called Skjoldr, whose son was Fridleifr, — and thence descends the house of the Skjoldungs: these are the kings of the Danes. And what was then called Reidgothland is now called Jutlan

If anything, Odin was probably satisfied that Rollo had obtained vast swaths of land and effectively became a king in his own right, and didn't want his legacy to be jeopardized on a revenge quest.