things have changed cause of her so there's a decent chance miss menace hasn't been able to develop the same relationship she had in the original timeline because of interruptions from MC's fiance at MC's behest and as such she might not be ready to stage a lover's double suicide yet (well, I'm assuming that's what she wanted it to look like in the original timeline, but even if it's not the case, the altered dynamics might mean they don't trust her enough to skip the poison testers yet which would in turn mean she wouldn't bother to use poison yet cause she wouldn't get the whole family)
...or she'll try to poison the family anyways and MC will save the day, causing her fiance to become more attached and paranoid due to the trauma, leading him back to the yandere path and back closer to the bad end routes for her