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. We ask Allaah to exalt his mention as well as that of his family and all his companions.
It the marriage contract took place while the conditions and pillars of marriage are met, then the marriage is valid. Not informing the husband about the rape does not affect the validity of the marriage contract. Rather, you should conceal what happened and not inform anyone at all. As regards whether or not you are sinful, if you were raped and were forced while you could not prevent it from happening and it was totally out of your consent, then you are not sinful. However, if you did that out of your own will and was not forced then you are obliged to sincerely repent to Allaah.
Finally, we advise all the husbands who faced the same situation like your husband, that if the wife appears to have repented and does righteous deeds, or that she was raped, then they should not hesitate to keep them as wives and treat them honourably.
Allaah Knows best.