Start with her husband

A normal woman will not risk engaging in an affair if she's happy in the marriage. In the beginning, you're not sure if Ilse let herself be fucked by Muller because she's not happy with her own marriage, or she feels compelled since Muller is the husband of her employer, or even both of them, to varying degrees. The information that you received tells you that she is not happy with her husband. The two of them work at the Muller estate, with Ilse as a maid and wet nurse, and her husband is the gardener. They got married not because they were in love in each other but because it's convenient since they're working at the same place.

The problem with the husband is that he's a drunkard and spend most of his time being drunk. That, and engaging in his philandering ways. Strangely, it wasn't the case during the earlier period of their marriage as people close to them described their relationship as normal. The change started when Ilse got pregnant, and her husband seemed to believe that he's not the father.

So, Muller is probably involved with Ilse even earlier on but that's not your concern.

You arrange a meeting with the husband, where you ask him about what it will take for him to divorce Ilse. At first, he's very **** to do it since such a proceeding will be expensive. But since you're the king, you have the power to advise the church to recognize the divorce. In the end, you arrange for him to be married to one of the prostitutes he's enamored with, and then the two of them will work at your castle, away from the Muller estate. You do warn him that while you're supporting him because you pity him, it's up to him to become sober. If he remains addicted to ****, he will soon find himself losing his new job and his new wife.

Ilse's soon to be ex-husband vows that he'll turn over the new leaf.

With that settled, you prevented the chance of the husband attacking Ilse or Muller in ****. So, onto the next step.