Yesterday, I discussed most of Poseidon’s close relatives. Today is his relationships with his OTHER relations, and with both those relatives and with deities from other pantheons.
Khronos and Rhea are Poseidon’s parents and the other Elder Titans are his aunts and uncles. This would make the second generation Titans such as Helios and Prometheus his cousins. However there were beings that were not among the Titans who were also related to Poseidon in some ways. This included the Elder Cyclopes, the Hundred-Handed Ones, Ouranos and Gaea, the other Protogenoi such as Pontos, Nyx, and Aether (among others). Generally speaking, His relationships with all of them was good — except for during the period of the Titan Wars, when Poseidon and his siblings were fighting for control of the Cosmos.
As for Poseidon’s relationship with deities from other pantheons, his relationship with his Roman counterpart Neptune, and with the Roman pantheon was not one that was problematic. In fact, there were no issues with any of the cultures around the Mediterranean basin. As he is Lord of the Oceans in the Hellenic pantheon and the oceans’ gods tend to be fluid in their natures, I personally think that Poseidon would have no issues with the gods of most cultures because humans probably all have similar gods.