If anyone is interested in playing a member of Eamonn's family, you need just to apply. He has a younger sister, but she's still 15 since they were in the same litter. I don't have any real rules for her besides the basic pack ones that apply, and I would like for her relationship with Eamonn to be at the very least civil. Sibling rivalry could be incoorporated, but ultimately it's up to you.
As far as Eamonn's mother goes, I see her as like a regal old dowager queen, very refined and sophisticated, but again, the finer details are up to you. She could be from the personality range of Betty White to Dame Maggie Smith. xD