OH, wow!
Off the cuff, I would say my immediate reaction is that I do not want to be involved in my sister's marriage, my daugher's marriage, my mother's marriage. It is not a good place to be for me and for them.
If I felt the child were endangered, I would speak to my family member and if I were really worried I would phone child services.
Does your brother come to you and the rest of the family for advice?
Is it good for your health and happiness to be involved with this?