Most of The Exeters are somehow named after an ancestor or someone important to the parent. Like Holly, she is named for her great-grandmother Maria and her aunt Roxy Jayne.
There tends to be a multiple birth in the family somewhere. In Holly's case; she had two sets of triplets and a set of twins. She also came from a set of triplets like her father, who was also a triplet.
My friend who roleplayed Roxy made her own twists to the character and added the children.