File a case of 498A with the police where she is living with her father. Also, file a case of maintenance u/s 125CRPC or even under the domestic violence act in the court of local jurisdiction. The solution would come out after some time. A second marriage can’t take place unless and until a divorce takes place. These issues are complex. You may file a police complaint on grounds of harassment and nuisance. You might require a witness for the same. Have a word with your brother and file a complaint.
You may also press for trespassing and coercion with torture. Under the law, a mother can sue her son and daughter-in-law, but a sister cannot sue her brother’s wife or her own sister. A single High Court judge in February last year quashed Kusum’s case against her sisters and in-laws under the law. Any physical violence, regardless of its severity, is qualified as cruelty and is sufficient to initiate legal action. Any verbal abuse, in terms of taunts, words, language, etc., is intended to cause mental torture.
If the marriage of your brother and his wife is not working out, there is only one option of divorce, and your brother should not delay in filing that. If her behaviour is not matching up with family members and she has been accusing other family members, it is not advisable to let her in your house. In the event of any forceful event, try to video record the whole incident. Tell your elderly sister, if she lives with you in a shared household, to file a DV case against your sister-in-law. Now DV cases can be filed against women as well. Tell your sister to get a protection order.
Reference: Hindu marriage act – 202100499-20210043-4235