Hello and welcome to my headcanon about Rhea having a romantic relationship with Wilhelm from Adrestia! Here are my crumbs of canon information to back me up.
Spoilers for FE3H Below
Rhea repeatedly references that Edelgard is a descendant of House Hresvelg. She goes so far as to say that Edelgard “humiliated [her] brethren” by standing against the Church of Seiros, and you simply don’t bring up a point like that over and over unless it has special meaning even generations later.
For those who don’t know, Seiros personally persuaded Wilhelm Paul Hresvelg to side with her against Nemesis, giving him her own blood. She’s done this before with a handful of people she deemed worthy, including Jeralt. So it wasn’t something she offered strictly as leverage to persuade him (although it probably played a part) by giving him a Crest of Seiros. She went on to help him found the Adrestian Empire, and the novel called The Feast of Decadence found in the Shadow Library implies it was a very popular rumor that she had a romantic relationship with Wilhelm:
A young man, staring at Emperor Wilhelm as he caught the saint and pulled her into a close embrace, suddenly beckoned me. He pressed his lips to my ear and whispered sweet nothings, mirroring Emperor Wilhelm and Saint Seiros upon the stage…
Given how Seiros consistently mentions Wilhelm around Edelgard after she declares war on the Church, I believe this rumor of their romantic relationship was true. Despite feeling betrayed by humanity for how the Agarthans turned against Sothis and her children, Seiros clearly trusted Wilhelm and elevated him to the role of Emperor.
Obviously, they didn’t have children together, but this history adds an entirely new level of offense to Edelgard’s perceived betrayal of Seiros. (I say perceived because there was human experimentation being carried out by the Agarthans with not a peep from Seiros’ leadership as all of Edelgard’s siblings were wiped out, as well as what happened with Lysithea, so it’s a very tangled situation.)
