All seats except extra legroom are free at OLCI, including twins. If you go to manage your booking it'll show you the breakdown of miles earned for each segment. You need 25k in a 12 month period for Silver. These are tier miles. Skywards miles are redeemable and expire in your birthday month 3 years from now, I'm not sure which you refer to as expiring, since seat selection has been chargeable for over 12 months so if this is your first encounter with it then you've likely not flown for over 12 months and thus have 0 tier miles.