How to help her?

My brother in law hit my sister when she wanted to live separately from her in laws, right now she has two kids and share the roof with father-in-law, mother-in-law, and brother-in-law with two kids of her own. She wanted to provide separate rooms for two girls who are 5 and 3 and my brother in law doesn't want to love serfdom his parents

How can I help my sister?