Sending Notice to My sister by his husband to come back to him

Hello My sister is living with me and my mother and father from last 8 month she came back from his husband house bec he is beating him not letting her go outside always spying on him always keep checking her phone wont let he meet relative they both had 1 boy age 11 now with his father my sister dont want to go again there in hell . We try everything with his husband to give her divorce she dont want to live with him and she also dont want the custodiy of child bec we want divorce from him but he is not ready to give it now suddenly after 8 month he sended today notice with wrong allegation and in end he told to come back home with in 8 days or i will take legal action against you and all the price and whatever happen will be on you. Sir madam kindly help what to do we have 8 days from today and she dont want to go back what shall we do we already told him we are agree to give divorce we dont want anything from you just give her divorce but still he is not agree on it and sended notic