How is Rhea's relationship with Thalia? In Titan's curse, she threatened Thalia but during the war, she was friendly. If a situation comes, will Rhae threaten Thalia or just talk it out? Is she like, 'I was only friendly with you because you were needed but I would've preferred threatening you'?
Also, Rhea didn't tell Thalia about her being reincarnated or about the existence of the multiverse. Will she ever tell her?
The relationship is neutral.
During the Titan Curse they were fighting, Rhea doesn't like showing her emotions and seemed indiferent about what happened to Annabeth when in reality Rhea was angry about it, her "indiferense" made Thalia angry. Hence why Rhea threatened her.
But otherwise Rhea would prefer for Thalia to completely stay away, she is friendly with her in casual talks or like someone would be with a stranger to keep things smooth.
Also Rhea will never tell Thalia about the multiverse or the reincarnation, that was something told to Annabeth, Nico and Grover because :
Annabeth : despite what it seemed Rhea cares deeply for the girl and wanted to be her friend, so telling her about the truth to solve their falling friendship was the solution to the whole problem so Annabeth could understand why Rhea is the way she is.
Grover : Their empathy link go both ways, and he saw Aidoneus for a brief moment and heard him and Poseidonas, he could also feel Rhea’s deepest emotions which is why he partially understands Rhea despite not knowing the whole truth, telling him just finished the other part of the picture for him.
Nico : he could feel death on her as if she died and returned back in a very weird way, he asked the question and he would have asked more, Rhea also likes him so she told him.