Your aunt's grandchildren would be your first cousins once removed, as the your aunt's children are your first cousins.
Your niece's grandchildren are your great great nieces and great great nephews. You are their great great aunt (if you are a woman) or great great uncle (if you are a man).
Your father's nephews and nieces are your first cousins.
They would be great nieces or great nephews.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
They are your great nephews and/or great nieces. Alternately, some people say grand-nephews, grand-nieces.
Your brothers granddaughter is your grandniece.
Most likely they would not be related to you
It says what relation are your brothers nieces to you.... It could be your child or your niece also causes you can have two brothers and it may be the other ones kids. Or of course it could be your brother's new wife kid by another man making her no relation.
You would be a first cousin by marriage, in other words, there is no direct relationship between you.
Yes, you would be the step-nan (or step-grandmother) to your granddaughter's step-brothers.
Your grandfather's brothers would be the uncles of your parent. That would make them your great uncle.
You would refer to them as your nieces, thought they are formally your wife's nieces. And they would normally call you uncle.
Your grandmother is first cousins with cousin. Your parent would be the second cousin of the cousin's son. That would make you second cousins once removed.