Your grand mother's niece is your parent's first cousin. Her children are your second cousins.
Hopefully, women who are grandmothers of the same children are friends, but in western culture they are not otherwise related. They are the mothers of the parents of the children
Sisters, mother, aunts, grandmothers, nieces, cousins, etc.
They can be parents, husbands, wives, children, aunts, uncles, family, a loved one, nieces, nephews, grandmothers, grandfathers, friends, and anything they set their mind to:)
Your niece's children are your great nieces and great nephews.
Your great nieces or newfews
Your mother's sister's children are your 1st cousins.
Maybe. They are only related if they share a common ancestor. There are lots of possibilities and some of them will be related and some will not.
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 mothers grandmothers children are each others siblings.
Nephews and/or Nieces
Nieces and nephews are the children of your siblings.