Your uncle's grandchildren are your first cousins, once removed.
They and your grand children are your second cousins, once removed
Nephews and nieces
They are your great nephews and/or great nieces. Alternately, some people say grand-nephews, grand-nieces.
No, nephews are not considered lineal descendants. Lineal descendants refer specifically to individuals who are directly descended from a person, such as children, grandchildren, and great-grandchildren. Nephews fall into the category of collateral relatives, as they are the children of a person's siblings rather than direct descendants.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
They would be great nieces or great nephews.
Your uncle-by-marriage's nephews might be you and your siblings. You are your uncle's nephew or niece.They might be the children of your aunt's siblings (your aunt being his wife), in which case they are your cousins.If they are your uncle's nephews by virtue of being children of his siblings, they are no relation to you at all.
Your son's children are your grandchildren.
Your mother's sister's children are your 1st cousins.
Your aunt's grandchildren would be your first cousins once removed, as the your aunt's children are your first cousins.
No, they`d be the second sisters great nieces and nephews
The common ancestor is your parents. Your children and your sister's children are first cousins. Your grandchildren and your sister's grandchildren are second cousins. Your great-grandchildren and your sister's great-grandchildren are third cousins.
Your great aunt's daughter's children (her grandchildren) are your second cousins.