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.
It is indeed a great calamity that evil comes from a person whom one feels safe and secure about. What happened from the maternal uncle of your husband is most likely due to being lenient about non-Mahram men entering upon women especially the relatives of the husband about whom there was a special warning from the Prophet and this is clarified in Fataawa 154373 and 127911.
The fact that this man is a maternal uncle to your husband does not mean that he is a Mahram to you if there is no other relation that makes him a Mahram; in this regard please refer to Fatwa 128305.
If we presume that he is a Mahram, then this happening from him is even more evil and more sinful. Anyway, he did well by asking your forgiveness, but he is obliged to sincerely repent to Allaah; the conditions of a sincere repentance are clarified in Fatwa 86527.
Also, you did well by forgiving him, so you should not inform your husband about the matter as it is an obligation to conceal the sins of a Muslim according to the view of some scholars. The prominent scholar Shaykh Ibn ‘Uthaymeen said while explaining Riyaadh As-Saaliheen: “It is an obligation upon a Muslim to conceal the faults of another Muslim and he should not spread it unless there is a necessity.”
Even if we presume that this is not an obligation, it is not wise to inform your husband about the issue to make him have hard feelings towards his uncle over a matter about which he had already sought your forgiveness and he might have repented from it. As-San’aani said in the book Subul As-Salaam: “If it is known that he has repented and gave up his sin, it is forbidden to disclose what he had done and it is an obligation to conceal his sin; this applies to the person who is not known to be corrupt or persistent in exceeding the limits of Sharee'ah.”
Allaah Knows best.