Janet is back from the quantum realm after a lifetime of absence from her daughters life. That absence from a young age prevented the westermark effect from forming between them (the innate aversion to incest between family members), and reuniting after so many years leads to genetic sexual attraction and the desire to make up for lost time. This results in a passionate and explosive sexual encounter between mother and daughter, which begins a long covert sexual relationship.
Hope Pym/Janet Van Dyne