A woman can not sue her husband for rape in Nigeria. A man can not rape his wife as the marriage between them is an express consent for both parties.

First, it's important to note that the exclusive rights to the children is bestowed on the parents (Father and Mother). In this instance the Father of the kids have the exclusive right to his kids to the exception of family members or in-laws.
Even though your late sister birthed the Children, the surviving Parent (Your Brother In-law) can insist on taking care of his children with or without help, assistant or allowing your family (including your Mum) access to them.

Kindly also note it is unlawful and criminal put any form of mark (either tribal or spiritual) on the face, body or any where on a child. The perpetrators of this act whether in the name of tribal mark or whatever are liable to be sentenced to prison. Section 24(1)(2) of the Child Rights Act, (Nigeria) 2003 prohibits making any form of skin mark on a child.

The said Section provides thus: “No person shall tattoo or make a skin mark or cause any tattoo or skin mark to be made on a child.

My advice to you and your family is to reason with your brother in-law on the need to be allowed access to the kids especially due to their young ages.
And if you guys feel the kids are not being taken care of properly or are being subjected to abuse and maltreatment, you can contact any of the various Child Welfare Offices to help out.