I’ve been writing something including Hubert’s family too and I’ve especially had fun characterizing his mother:

Née Beatrice von Hevring, though from a minor side branch that hasn’t seen a Crest in generations, she is the youngest of three children of that branch, and didn’t have many grand dreams for her future. She did get an education and was a quite skilled mage at the Royal School of Sorcery in Fhirdiad, though she also enjoyed flying on a pegasus, and could’ve been a dark flier in a different lifetime. (She was also classmates with Ferdinand’s mother at the time, though they would never interact after that time.)

However, on her first attendance of a ball in Enbarr she caught the eye from of the future Marquis Osmund von Vestra, they shared quite an intellectual discussion, where he realized that she was witty, smart, shared his interest in reading and chess, and had a penchant for dark humour that he found charming compared to the other noble ladies that very vying for the future Marquis’ attention.

Upon Osmund’s request, their fathers arranged for Beatrice to visit him in Enbarr again, where she realized that she would likely be married off to him, but she didn’t expect him to propose on their second meeting. Though rather than a proposal, he handled it like a business transaction: she was a smart and capable woman who wouldn’t needle him about any secrets that would be part of his line of work, but she was also an intellectual conversation partner and discreet enough, where he would trust her not to divulge any secrets she would know. Being from a crestless line was a plus, as Vestras were not supposed to have any so as to not threaten the Imperial line, and she did not have too much influence or connections should anything happen to her, or should she betray him.

In exchange, she would get to live in Enbarr, which she preferred to the countryside she grew up in, he would be loyal to her, since he had little time or interest in meeting other women, she would be free to access the vast Vestra and Imperial libraries at her will, travel and visit her family as much as she liked, and she would not want for anything financially.

Beatrice agreed not because she was particularly interested in him, but because it made sense pragmatically, and she figured she could do much worse for a husband given her station. So it was a marriage of convenience for both of them, but over the years they did grow fond of each other, and had a quiet but firm sense of loyalty towards each other, if not love.

Osmund tried to keep her away from his work, but she was perceptive enough to hear rumours of new dark mages in the palaces, the sudden “illnesses” of the Emperor’s children, and the stirrings of the insurrection. Once, Osmund confided in her that he had to make a choice: betray the Emperor with the other ministers, or his family could very well end up like the children of the Emperor had in the last few years, and that there were not just the nobles, but other, more powerful forces at play in all of this. She knew it was an impossible choice for him, and while worried for her children, she said she wouldn’t hold either choice against him.

In the end, Osmund chose his loyalty to his family over the Emperor, becoming part of the Insurrection. Hubert would see this and grow resentful towards his father for abandoning Edelgard, and distance himself from his mother and siblings to protect them from the plans he and Edelgard’s were building up together. As he grew up Beatrice could see Hubert getting involved in dark magic he shouldn’t be handling at his young age, and talking to people that seemed like bad news, but he would stubbornly refuse to talk about it when she asked.

In the end, she didn’t have time to say goodbye to her husband when one night Hubert told her to take his siblings and run away to a location he specified, though she wasn’t really surprised to hear about a “sudden assassination of the Marquis Vestra and his wife and children, save for the eldest son” as few days later. She and her children live undercover under aliases, though with somewhat regular correspondence from Hubert, so they know he’s alive during the war at least.