The great aunt is the great niece's baby's great-great aunt.
Second cousins.
If your great uncle married into your family some of his nieces and nephews are your aunts or uncles (or your parent) and some are not related to you at all.
Your daughters are great aunties if their nieces have kids. Your daughters kids are your grand children.
Your borther-in-law is your grandchildren's great uncle. They are his great nieces and great nephews.
You would be their great-granduncle or great grand aunt, and they are your great-great nieces or great-great nephews.
If your great uncle is the brother of one of your grandparents, his wife is your great aunt but their nephews and nieces onher side are not related to you. Those nieces and nephews (and their children the great nieces and great nephews) have no ancestor who is also one of yourancestors.
Your great grandfather's brother is your great great uncle.
Your great nephew's children are your great great nieces and great great nephews. You are their great great aunt or great great uncle.
Your great-aunt
Your great nephew's children are your great great nieces and great great nephews. You are their great great aunt or great great uncle.
Your niece's daughter is your great niece. You are her great aunt or great uncle, depending on your sex.
Your niece's children are your great nieces and great nephews.