They really aren't your blood nieces, but most people don't care and let the children meld into the rest of the family as one of their own. If this was a legal issue (which it isn't) then no, they really aren't your nieces. They would only be your legal nieces if your brother fathered them. Children are innocent and have no control over adults, and most families have something like these circumstances so it's best to treat them as you would your own nieces, which I am sure you have every intention of doing.
Nephews and/or Nieces
You might want to call him your "former brother-in-law," especially if there are children who are your nieces and nephews. If you really like him, you can continue to call him your brother-in-law, if you want to.
Your brother's wife's kids would be your nieces or nephews.
Your brother's child's children are your grandnieces or grandnephews.
If your brother (or your sister) has children they are your nephews (males) and nieces (females).
Your brother's children would be your nieces and nephews.
Your brother-in-law's children are your nephews and nieces.
Children of your siblings are nieces and nephews. Children of your spouse's siblings are still called nieces and nephews, but you are not related to them by blood.
Your sister-in-law is either the wife of your brother or the sister of your spouse. The children of your brother's wife are your nieces and nephews, since they are also the children of your brother. In some cases they may be your sister-in-law's children from a previous marriage. Then they are not related to you at all. However, if they are very young, it may be convenient to call them your nieces and nephews anyway. The children of your spouse's sister are always your nieces and nephews.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
No. He had a brother patrick who married but had no children.
Your sister's husband is your brother-in-law. Your brother-in-law's brother is not related to you and neither are his children. But he is your sister's brother-in-law and his children are your sister's nieces and nephews.