All the six pandavs were not the biological sons of King Pandu,

All the six pandavs were not the biological sons of King Pandu, whereas the Kauravas were born of Dhritarastra and Gandhari. Then how come the claim of throne by Yudhisthira was justified? Suyodhan must be the rightful king then. Even dhritarashtra,pandu and vidura were not biological sons of vichitraveerya. Should we make bhishma as king then ? Pandavas are 5 not 6. Pandavas are king pandus sons. Karna don't come in Pandavas. He is kounteya son of Kunti. Niyoga was accepted practice. Pandavas were born same way in which dhritarashtra,pandu and vidura were born. Dhritarashtra was never crowned as king of Hastinapur in his entire lifetime. He was just a caretaker. So Yudhisthira by both traditional Inheritance right as well as merit a rightful heir. Duryodhan by any argument or logic was not a heir. Dhritarashtra has said this number of times himself.