She's terrified that he was such a good judge of character/planner/maybe manipilator, because he was able to find our MC who, with no blood relations, is willing to sacrifice everything for his children.
Somehow anyone could relate to her fear.
I'd be scared too, someone like that while they may seem kind is a calculated and cold person who works only for his own gain. MC benefited from being taken into his household but she was also burdened with a mountain of other issues. Uprooted so young and made to become the "mother" of children who are basically the same age as her. It's no wonder she failed in her first life, she was a child, and now she's here as a 20 year old in the body of a girl.
Wait, i don't get it. Why is Lucrecia scared of her older brother?