Nah, I get it. There reaction was quite sudden, and unnatural for someone who is said to have a traumatic experience with a person. But I’m guessing that the old Senianna wasn’t very close to the two brothers, and never tried to approach them. So when things goes south she had no one. Since the new Senienna has proven the old ones innocence, she is viewed as more trustworthy now and pitiful at that. Anyone is nice when the other person is pitiful. In addition to that they are blood related so they were chill to accept her. Probs a medieval fantasy type of shit goes down as to why they live in the same house but never interacted.

It feels really weird to have them getting goody-goody towards Senianna, like heck, they only like her when she is FINALLY useful for them, like wtf? Literally a bruh moment. I just cannot. If she is gloomy and suicidal, they leave her alone, with some wicked bitch eww to care for some suicidal girl with issues (see this one, like, I cannot... gramps is totally frustrating AF and now he's getting all warmed up and caring Senianna as if she's his prodigal granddaughter. Bullshit. NO.)
At least Senianna's quite forgiving. Lancelot is utterly creepy. Totally, in my opinion. Gawain is just no... Seeing a tsundere is already annoying enough but he let his trauma from Senianna's (IIRC, the OG Senianna publicly shaming Gawain about their mother abandoned him or something when they were children) let it go.
That's all I want to post but feel free to correct me if I'm wrong.