Hmm. I like the initial idea of an isekai character knowing what's going on, and ending up interacting with the big baddy or whatever, but wow this setup is contrived.
They could have just left it at 'MC takes brothers place to avoid plot point x' but instead there's a runaway mom with a debt with literally no other info offered on that situation (how did she get the debt? Why is it apparently to great that a former head of guard and noble can't handle it? Etc), and instead of running away or something to avoid the fate of her brother becoming evil, she spends her fast forwarded early years teaching him to love. To which he responds by running away with a random girl, which *directly* condemns his sister to a death sentence.