All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If your husband has really molested his two sisters, then he has indeed committed a grave major sin, as it is a very abominable act for a man to molest his female Mahram relatives. Even if he did so with any non-Mahram woman, it is forbidden, but it becomes even more forbidden and more sinful with Mahram relatives. It is expected from such a person to protect the dignity and honor of his Mahram female relatives and not be the one who transgresses their honor and dignity.
Indeed, feeling ashamed is not enough for your husband; rather, he is obliged to sincerely repent to Allaah fulfilling the conditions of a sincere repentance that are mentioned in Fatwa 87903. Moreover, he should perform good deeds as much as possible hoping that Allaah will forgive him, as Allaah is The Al-'Afuww (The Pardoner with wide forgiveness) and the Al-Ghafoor (The Forgiving in abundance).
Besides, it is an obligation on you to hate the acts of disobedience that he did, as this is getting angry when the limits set by Allaah are transgressed.
However, if your husband had repented from that abominable act, then praise be to Allaah, as the one who repents from sin is like the one who has never committed a sin. But if he does not repent, then you should advise him and remind him of Allaah’s punishment and wrath, and you should endeavor your utmost in order to rectify him so that he would repent. If he refuses, then it is no good for you to stay with him because such a person cannot be trusted with matters of honor and dignity. Hence, you may ask him to divorce you and if he does not accept, then you should take the matter to one of the Islamic Centers or to an Islamic judge in a Muslim country.
Allaah Knows best.