But considering Robert took the Throne based on his grandmother Rhaelle otherwise why not Ned/Jon/Holster, wouldn’t descendants of Egg’s sisters be the next legal heirs? Or the Hightowers, Martells, Penroses, Velayrons etc

To clarify: Robert took the Iron Throne because the rebel alliance acclaimed him King before the Battle of the Trident and then carried that claim to victory at the Trident and King’s Landing. They chose him for a couple reasons – he was charismatic, he had some Targaryen blood, there was the whole symbolism of Rhaegar’s abduction of Lyanna – but they weren’t required to. 

And once he’s on the throne, the terms of the next legal heir shift to his relatives. Hence Stannis being next in line if Robert has no kids.