From last 4 years ‘everyday’ your sister was being beaten up and put on ‘house arrest’ by her in-laws including her husband and ‘you’ waited for situation to improve is what you say to us.
In my opinion such alleged facts are not possible and even if so you are more to blame then her In-laws and her husband to have put a sister in such situation from last 4 years.
1. The first remedy is to immediately re-call your sister and kids to her natal home and only after bond of good humane behavior from her in-laws and her husband taken, she should be sent back to her matrimonial home.
2. Till such time support her and kids in her natal home which is what you as her brother should immediately do if daily beating is silver quality fact and not some exaggeration.
3. Till spouses family matters are in Civil law domain such alleged to be facts family matters can be resolved and the moment police and or criminal court enters couples family matters your sister’s matrimony will never get saved, hence donot react yet ‘respond’ accordingly.
Hence if you co-joint read above three paras and pay more attention to para 3 you as part of her immediate blood relation succeeds in saving your sister’s matrimony otherwise leaving her beaten daily from last 4 years itself is greatest mind, body and soul damage a brother or relative to a sister has already been caused by relatives of her.
4. Resolvable mediation are possible in Civil cases such as ‘mutual consent divorce’ and for the same ask her if she can pull life with a divorcee tag if given a chance to empower her while being re-called and taken care of at natal home so as to stand on her own feet and seeing life afresh.