Do not do it - it's not doing any good to anyone
you are destroying several families
your sister's family- your dear sister and her poor 2 kids will have miserable rest of their lives
your family-it will not be long your relationship will be disrupted too
your parents family- you will be the qianguzuiren in your family tree
Your brother in law is not sunzhongshan or jiangjieshi
instead he is a shame less. Irresponsible person