How can deeds be accepted after repenting from backbiting the husband's family, given the ongoing animosity and their unwillingness to reconcile, and is it true that deeds are not accepted in this case?

If you spoke ill of your in-laws without right, then you have done well by repenting and seeking their forgiveness. It was incumbent upon them to forgive you, due to the great merit in pardon. They should be reminded of this, and it is better that the reminder comes from someone they respect. We advise you to frequently supplicate for their well-being and to mention their good qualities.

The hadith (definition) narrated by Muslim from Abu Hurayrah states that the Prophet, peace and blessings be upon him, said: "The gates of Paradise are opened on Mondays and Thursdays, and every servant who does not associate anything with Allah is forgiven, except a man between whom and his brother there is animosity. It will be said: 'Delay these two until they reconcile.'" This hadith warns against the persistence of animosity between Muslims and encourages its cessation. Since you initiated reconciliation and sought forgiveness, we hope that this warning does not apply to you, for whoever apologizes has fulfilled their duty. You must also be diligent in guarding your tongue from evil speech.