Sister committed suicide because of husband & family

My sister got married in 2004, conceiving pre-marriage after a relationship of 4 years and was a victim of domestic violence. She committed suicide in the year 2009 claiming her husband and his family members' name in the suicide note. We didn't take any legal action considering the kid will be parent-less.

She also mentioned in the note that she wanted her daughter to be handed over to her father. So she was staying with us. Now, after 3 years, his father wants his daughter back and took her without informing my father.

Now we realize that they shouldn't have been spared so can I show the suicide note now and file a case against him and his family?