Judging by other things said so far in the series, the strength they're referring to is the Asmodian deal the father and brother have going on. Faster healing, stronger body than normal humans. It's possibly a "we might not even die if you try to kill us" sort of thing more than something she could train for. Like the injury topic when they went for a ride on the horses. The brother was apparently unaware that falling off horses typically causes injury.
I was hoping she was going to be like "Then I'll have to become stronger!" so she can go to school, and then a training montage starts. Cuz frankly a protagonist that's too weak is pretty useless.