One of the common elements of all the Dharmic Religions is, that they have the same deities/Characters, whose story follows the same general outline, but in details they are very different.

The answer is yes, It's true, but only acc to the Jain tradition, not the Vaiṣṇava traditions or any other tradition of Hinduism.

And acc to the “Kṛṣṇa was the cousin of Neminath” story, Kṛṣṇa went to hell after death and is still there. Kṛṣṇa's greatest foe was Jarasandha, and it was Kṛṣṇa who slayed Jarasandha, not Bhīma.

But this contradict with other Hindu traditions.

Acc to the Jain tradition, There are 63 Śalākāpuruṣa

• twenty-four Tīrthaṅkaras (spiritual teachers),
• 12 Chakravartins(Universal Monarchs)
• nine sets of triads of
• Balabhadras or Baladevas(Gentle heroes)
• Nārāyaṇas or Vāsudevas (Voilent Heroes)
• Prati-Vāsudevas or Prati-Nārāyaṇas(Anti-Heroes or Villains)

One of these triads is Kṛṣṇa as the Vāsudeva, BalaRāma as the Baladeva, and Jarasandha as the Prati-Vāsudeva. In each age of the Jain cyclic time is born a Vasudeva with an elder brother termed the Baladeva. Between the triads, Baladeva upholds the principle of non-violence, a central idea of Jainism. The villain is the Prati-Vāsudeva, who attempts to destroy the world. To save the world, Vāsudeva-Kṛṣṇa has to forsake the non-violence principle and kill the Prati-Vasudeva(Jarasandha). In Jain Mahābhārata, the main battle is not the KuruKṣētra War, but the fight between Kṛṣṇa and Jarasandha who is killed by Kṛṣṇa.

Kṛṣṇa loses battles in the Jain versions, and his gopis and his clan of Yadavas die in a fire created by an ascetic named Dvaipāyana. Acc to this, a seer named Dvaipāyana got offended by Kṛṣṇa's intoxicated son Sāmba, then he invoked AgniDeva, and burnt entire Kingdom with its people. BalaRāma& Kṛṣṇa turned towards Mathura, where Kṛṣṇa gets severely wounded by the arrows of Jara, who mistakens Kṛṣṇa to be a deer. In his last moments Kṛṣṇa develops hatred for Dvaipāyana.

This is similar to the version mentioned in the Mausala Parva of Mahābhārata, but in details it's very different.

Another of this triad was Rāma as the Balabhadra, Lakṣmaṇa as the Nārāyaṇa, and Rāvaṇa as the Prati-Nārāyaṇa. Acc to Jain Rāmāyaṇa, Sītā was the daughter of Rāvaṇa, it was Lakṣmaṇa who slayed Rāvaṇa. In the next birth Rāvaṇa will be a Jain Monk and Sītā will his disciple.

The Balabhadra goes to heaven, Vāsudeva goes to hell, on account of their violent exploits, even though they did these in order to uphold Dharma. When BalaRāma saw that Kṛṣṇa was suffering in the Hell, he tries to save Kṛṣṇa.

This is similar to Pāṇḍavas(except Yudhiṣṭhira) going to hell for breaking the rules of War, even for upholding the Dharma. And when Yudhiṣṭhira saw his brothers suffering in hell, he tries to save them.

Kṛṣṇa receiving knowledge from neminath, acc to Jains. This might be similar to what is mentioned in Chhāndogya Upaniṣad 3.17.6, where Kṛṣṇa is depicted as a student of sage named, Ghora-Aṅgirasa; he receives knowledge from the sage, & becomes desire-free.

Also In the Mahābhārata(in the ‘Anu-Gītā Upa-Parva' of the 'Aśvamedhá-Parva'), Kṛṣṇa tells how he amassed this great knowledge of Gītā is, by interactions with many learned men, and by his own meditations. Kṛṣṇa also mentions about the knowledge he obtained from a certain Brahmin.

The first Tīrthaṅkara of Jainism, ṚṣabhaDeva Ādinātha, is mentioned as one of the 24 Avatāras of Viṣṇu in the Śrīmad Bhāgavata Mahā Purāṇa. His son Bharata is also mentioned in the Bhāgavata Purāṇa.

Acc to Jain tradition, ṚṣabhaDeva and Ikṣvāku are the same person, which makes Rāma as the descendant of ṚṣabhaDeva acc to Jainism. But acc to Vaiṣṇava traditions, they both are two different non-related persons born in different Manvantras. ṚṣabhaDeva in the 1st Manvantra and Ikṣvāku in the 7th.

Similarly the Jātaka Tales of Buddhism also mention about Purāṇic and Itihāsa's characters, whose story follows the same general outline, but in details they are very different. The Ghata Jātaka tell about Buddhist version of Kṛṣṇa's story. The Daśaratha-Jātaka tells about the Buddhist version of Rāma's story.

Rāma & Kṛṣṇa are also mentioned in the Sikh text Chaubis Avtaar.

So basically all the Dharmic Religions have the similar concepts, characteristics, similar outline, but in details they are very different.

In the long history of Dharmic Religions, they have Incorporated various elements from each other and mixed some additional features of their own.

However, I beg to differ from Harshil's answer. This is not a propaganda to attract Jains towards Śrīmad Bhagvad Gītā. It is SOLELY a Jain concept, not anyone else's. I haven't seen anywhere(except Jain scriptures), someone named SamudraVijaya, to be a brother of Vasudeva(Kṛṣṇa's father), and his son Neminātha to be a cousin of Kṛṣṇa and narrating him the Gītā.

So It's completely a Jain concept.

The brothers of Vasudeva as mentioned in the Harivaṃśa Parva of the Mahābhārata are Devabhāga, Devaśravā, Ānaka, Sṛñjaya, Śyāmaka, Kaṅka, Śamīka, Vatsaka and Vṛka. The Śrīmad Bhāgavata Mahā Purāṇa mentions Nanda Baba as the half brother of Vasudeva.

Click on the below link to know more about the differences between Jain Rāmāyaṇa & Mahābhārata and the Vālmīki Rāmāyaṇa& Vyāsa Mahābhārata.

How different are the Jain Ramayana and Jain Mahabharata from Hindu narrations?

Footnotes