Rein

I had an abusive mother. One who was neglectful at first, then physically, mentally, emotionally abusive and then back to neglectful. Eein is NOT that at all. She tried and thats more than many mothers out there do. Giving the child to the imperial family is whats best for him if she feels she's emotionally unable to care for the child. Not to mention that the child would have been taken from her anyway, so distancing herself was probably a mercy to the boy if he was eventually going to leave anyway. She couldn't hide him forever.
Also, Rein has extensive trauma she hasn't dealt with and,unfortunately, Wilhelm triggered that trauma in the worst way possible. Just because it doesn't amount to betrayal to you, or because it doesn't seem all that important to you doesn't mean it isn't for someone else. Have a little sympathy if you can't have empathy. Rein lost EVERYTHING in her first life and then in her second, she thought she had everything only to be betrayed by the one person she thought would never betray her. And not once, but twice, because she forgave him the first time. He literally lied as he told her he was telling her the truth when all she wanted was the truth. If that isn't enough for her to lose all trust in him, then I don't know what is. I would act the same way because of my trauma as well. I have a very visceral reaction to those I get close to betraying me. It literally tears me apart, which is why I have very few I get close to at all.
Also, I'm really bummed that Dietrich has someone else. I feel like the author didn't know how to resolve him so she did the amnesia thing. Seemed like a cop out to me. I'm glad he's alive, don't get me wrong, but I'm bummed the amnesia is sticking. I am glad to see he's happy and hasn't changed, though.
And yes, the child has done nothing wrong and he doesn't understand. Even Rein knows that. But just as betrayal cannot stop love, love cannot cure trauma. Part of me still loves my mother. But I don't love what's really her. I love the glimpses she showed me in public when she acted like my mom. At least Reinhardt is a good person. She genuinely loves the boy, you can tell, but she's haunted by him. She wants him to have a good life but knows she can't give it to him. Rather than possibly having that love turn to resentment or to further abuse him, she'd rather give him a chance for that somewhere else. Though, to be perfectly honest, I would have given the child to Dietrich and had him raise the child as his own instead and just played the doting mistress. But that would have only hurt Dietrich and Rio when the imperial family took him. . . There really is no win in this situation. There's only the lesser of the two evils.