Don't know how many of the players here still check, but I'm going to list out the eligible girls for Prince Selwyn, who is currently around 11 years old. Let me know what you think.
Ruth Marbrand (same age)
Sybil Hightower (one year younger)
Joyra Karstark (one year younger)
Alayne Waxley (two years younger)
Alexa Varner (two years younger)
Ellette Hunt (two years younger)
Visenya Naharis (four years younger)
Merel Hunt (four years younger)
Elisebeth Varner (five years younger)
Tasha Harlaw (seven years younger)
Laya Dondarrion (seven years younger)
Apparently Victoria Dawnrose is also the same age as Selwyn, but I didn't see her on the family tree.
Right now, I'm thinking no Hightowers or Dondarrions because they were the most recent rebel leaders against Rodrick. The others are mostly minor nobility, especially the Hunts and the Marbrand girl. However, the older Varner girl might be a good way to hedge Rodrick's bets in the Far Reach and gain their allegiance. Alexa Varner also has a Royce grandmother (Anne Royce Varner).
A Harlaw queen would be hard for most mainlanders to accept. Visenya Naharis is from the Velaryon family, and doesn't bring power or a useful allegiance. At this point, I'm thinking that the older Varner girl is the best idea. Any thoughts?