#Don't even get me started on my headcanons surrounding the relationship between Medee von Vestra and the Kingdom of Faerghus.

[malsfefanfics]

why can't hubert's siblings be recruited into blue lions?

(Yay! An OC ask! Thanks so much!)

Aside from familial loyalty? To put it simply, the culture of Faerghus, the expectations of Noble Society, and the Kingdom's connections with the Church of Seiros would make it impossible for them to thrive. And joining the Church itself is completely out of the question.

Tancred is chronically ill, his magic being too strong for his weak body causing issues, and currently can't speak vocally, relying on Sign Language and writing to communicate. With what we've been presented with in canon regarding how children in noble families are treated, Tancred would greatly suffer growing up in the Kingdom of Faerghus, because its current culture is hostile to chronically ill, disabled children like him. It would take a lot of societal restructuring to ensure he'd he able to live a happy, healthy life under Kingdom rule. Restructuring that I don't think would be near base level needs in his lifetime. Tancred also doesn't have a Crest, and doesn't particularly want one either. But because he has no Crest, he'd not be seen for the same worth as a child with one by most of the Kingdom's nobles. And with conditions as they are, I don't think most commoner families would consider taking him in over the period of the war, either. The little one has needs that just can't be met by current Faeghus society.

Sure, he'd probably be welcomed into the Church, but he'd most likely end up in Abyss once he got older due to his disabilities, if he didn't find his way down there before age 10 first due to potential abuse from some of the Church staff. Seriously, have you listened to some of the dialogue from them lately regarding the people of Abyss? Mini-Bert just wouldn't last long at the Church up top or down below. Not unless the Ashen Wolves, Rosamund, another noble from the three houses, Alois, Seteth, Hanneman, or Manuela looking out for him.

As for Rosamund, she is skeptical of anything regarding the Church and the Goddess due to, what I guess you could call, a mix of her layered crisis of Faith as well as her anger over their suppression of basic information like crop rotation and the printing press. She also wouldn't have the patience to deal with some of the less savory parts of the Kingdom's beliefs, especially regarding the differences in raising children with a Crest vs raising ones without a Crest. The last thing you'd want to do is to take her to Galatea territory because she'd be furious at the state of things there. There would have to be a lot of changes going on pre-timeskip before she'd consider even entertaining the idea of joining the Kingdom. And they'd have to happen faster than the rate Dimitri would suggest. Not to mention I don't think she'd be very welcomed in the church for long because she just loves learning things and applying what she learns to her own life (a trait she gets from her mother). She would absolutely start using anything she learned from the secret library if she thought it would be useful, and I don't think Rhea would like that very much. At best, she'd constantly butt heads with the Archbishop. At worst? She'd make herself Church's Biggest Enemy #3/4 after Edelgard, Hubert, and potentially Byleth.

To be honest, I think Rosamund would relate a lot to Claude's ideals and visions of the future for Fodlan, and would see him as a fairly viable option. It'd be hard, but if the cards were played right, I could see her being persuaded into joining the Alliance. Yes, it has it's problems too, but compared to the other factions? If Claude can make the changes Edelgard also wants to see happen, and he's able to prove he can do it better, she could make the hard decision to join his side. If nothing else, then to at least ensure her baby brother's future by being on the winning team. As much as she loves Hubert, if it came down to a choice between her brothers, she'd choose Tancred. (And Hubert wouldn't have it any other way).

So in non-CF routes, Tancred would stay home with mother, and Rosamund would join Claude. So long as Claude wins the war and makes good on his promises, Rosamund will follow. Otherwise, they stay on the side of the Empire.