@XNovalX
I'm betting the original novel explains this more clearly (unfortunately it's not translated), because shorter manga adaptations like this often misses some key details:
1. her family is ridiculously powerful, so she probably can get his sister killed easily: either covertly or legally through death penalty
2. she does seem to have a valid reason to be suspicious of his sister, and judging by her tone,
she is probably planning to investigate the sister seriously (
not falsify evidence)