The Stiltons are a mouse family in New Mouse City which consist of:

William Shortpaws
Mr. Stilton
Mrs. Stilton (nee Shortpaws)
Thea StiltonGeronimo StiltonMr. StiltonMrs. Wugsy/Spring (nee Stilton)
Trap StiltonBenjamin StiltonBugsy Wugsy / Pandora Spring

It is unknown how Petunia Pretty Paws is Pandora Spring's niece, because Petunia would have to be either a Stilton or a Spring. However, it is possible for Pandora Spring to be a great-niece to her, however Petunia doesn't seem that old to be Geronimo's aunt.