To darkstar and Bryan :

Limiting spells to these requiring 5th level or less sounds OK., but what if somebody has minor access to sphere with realm spell AND can cast within it spells from 4th lever or greater ? Take a look on paladins - they can cast priest spells from levels 1..4. If we treat (blooded) paladins as priest, we must assign them other realm spell "levels" ...
I'm using Player's Options - there ("Spells&Magic") minor sphere of influence is treated as more costly, not more limited. Look, if a deity grants minor acces to some sphere, it simply means that this deity has less interest in that sphere (area of control, etc) than in other. In S&M, magic has been changed to point system, and spells from minor sphere cost twice. So, why not raise price of realm spells ?
Multiply cost in GB i RP of ALL realm spells from minor sphere of influence by two (or three) - this is fair for [N]PC and gives them more versality in casting spells than limiting them to just a few spells. Note that in BirthRight levels of all spells are slightly strange (first level "Rain of Magic Missiles" affect about hundred targets at once - someone thougth about "Rain of Meteor Swarms" ?) - so better give access to more of them for greater cost than limit them at all.

P.S. Paladins CAN or CAN NOT cast realm spells ? They can gain regency from temples and often have better connection to their god than priest ...

Gargamel