Bhīṣma was elder brother of Dhṛtarāṣṭra's father. He was so old. But he was... Actually, the kingdom belonged to Bhīṣma, but he remained a brahmacārī, he did not marry. There was no issue of Bhīṣmadeva. Therefore his nephews, Dhṛtarāṣṭra and Pāṇḍu, they were inheritor.
Now, after the death of Pāṇḍu, there was conspiracy. Dhṛtarāṣṭra wanted that "Actually, this is my kingdom. Now, somehow or other, I could not get it. Now my brother is dead. So if I do not inherit, why not my sons.?" This was the politics. Politics are always there, and enviousness, jealousy. This is the nature of this material world. You cannot avoid it.