What relation is.....

OldMeg your great niece’s mother or father must be your niece or nephew, so your DD is your great-niece’s first cousin once removed.
Your grandchildren are your great-niece’s second cousins.

First cousins share a grandparent.
Second cousins share a great grandparent.

The removed bit indicates the generations. So once removed is a generation down.
Twice removed is two generations down, etc.
Hope that helps

I think I agree with Janeainsworth. Took me a while to work it out.

Thank you Jane that seems logical. ??

Just been through that conundrum in my own family. I had my nephew from NZ staying here with his wife and two daughters. Since he is my sons' first cousin, my sons' children are his first cousins once removed; and their children are second cousins. In other words, my sisters' granchildren and my grandchildren are second cousins.

Armed with all this information I look forward to being able to explain the various relationships ? I think!

There is a chart online somewhere which explains all this.
I used it for reference when I was doing my family tree.
I will have to try to find it, I will post if Ido.

Just ask Google about family relationships, it is called very grandly consanguinity.