Before, she had thought about letting Amelia Walker take contraceptives, but she couldn't intervene in their marital affairs. As for administering drugs, it was too risky, like with birth control pills that had to be taken daily. Could she guarantee she would never be found out? She also considered secretly giving Amelia a little of the highly toxic cancer treatment drugs; just a little within six months could lead to a deformed fetus if she got pregnant. But she couldn't bring herself to do something so despicable.
After much consideration, she had to strike at the source of the problem. She did all of these things for two reasons: one was to vent her anger at Ruby Carter, and the other was to take advantage of this opportunity to drive Amelia away from Sebastian Carter before she got pregnant. That way, she would have no chance to get pregnant and no chance to turn her fortune around.