Karna was son of Surya and Kunti and he was the eldest Pandava as per law of that era. Yuddhisthir was son of Dharma and Kunti. Yuddhisthir was Dharmraj and he never thought ill of anyone. But when he and his brothers were denied even basic survival, he had to fight for his rights.
Karna always sided with Duryodhana because he was the one who gave royalty to Karna and made him a King. Karna was indebted to Duryodhana although their relationship wasn't of equals and Karna always prioritized himself over Duryodhana. Obviously he would side with Duryodhana even when Krishna and Kunti tried to call him back to his family because even though he had flaws, he wasn't a traitor and couldn't forget the respect and the graces of Kauravas when he was just a son of Charioteer. That doesn't mean he wasn't wealthy, Karna was very wealthy and his family could afford to send him to the Gurukula of Dronacharya but in the end Kingship was given to him by Duryodhana.
When Yuddhisthir got to know that Karna was his elder brother, he was devastated. He felt extreme grief for killing his brother through Arjuna. Even Karna said that to Krishna that if Yuddhisthir got to know the reality, he would give the throne to Karna and he would give it to Duryodhana to pay his debt of making him a King and it would be unfair as Yuddhisthir was the only right person to rule Hastinapur.
Yuddhisthir was always sad remembering Karna and he said his death pained him more than even Abhimanyu and Uppandavas. When he went to heaven he wished to see Karna along with his other brothers and Draupadi and only felt relief when he saw Karna with Adityas. Whenever Yuddhisthir used to see feet of Karna, it reminded him of Kunti and his anger would calm down but he never understood the reason behind it till the revelation of the truth.
Karna had no love towards Pandavas but he recognised the qualities and just nature of Yuddhisthir and considered him fit to rule Hastinapur. Meanwhile Yuddhisthir even when never ever sharing any warmth with Karna yet mourned and loved him when it was revealed he was his brother.
At that time an elder brother was considered equivalent of a father and despite all the hatred from Karna’ side atleast Yuddhisthir held no malice for Karna.