Apparently, I may have been talking bollocks. Unusual, I know. I've just been told, by the Wife, who is an identical twin type wench, that this is how it works.....
An identical twin birth (one egg splits) is a totally random thing, and a female one is not any more or less likely to have any twins herself.
A fraternal twin (multiple eggs), is two non identical babies at the same time, and a female one is more likely to have another fraternal pair of twins herself. She also has the same random chance of an identical twin birth.
I think that's it anyway.