@waiden Nian is known for being distant with people. She does socialize, but in a way similar to Mostima, she talks but there is no feeling of closeness from her. That is not true with Lava, the only being she is actually close to, even more to her siblings as we have seen.
Why that is yet to be known, but for now the fandom took it as shipping material.
Not that I mind, just explaining the facts here.