The problem of constant orientation– That is, if an effect present in a creature, object or area always confirms or denies if the objective is valid and, if it is denied, something that happens (dissipating, suppressing, exploiting, etc.) to that objective or effect is not it is mentioned, as far as I know, anywhere as a general rule. (Specific effects may mentions the constant orientation, but, for example, the ability to share spells from a relative does not make it or the third level Sor / Wiz spell primary form [trans] (Compendium of Spells 161).)
the Frequently asked questions about D & D-With what some people oppose- addresses this idea, albeit indirectly:
Yes, while under the effect of a spell that depends on the type (such as hold person), my character is transformed into a different type of creature by polymorphous organism, Does the effect of the spell remain?
Yes. A spell only checks if you are a legal target when it is cast. If you become an illegal target later (as through polymorphous organism spell), the spell remains in effect. (87)
(I am not aware of the origin of this question). Without any of the rules that indicate a constant orientation is one thing and this decision that, for this specific case, anyway, It is not one thing, this DM feels comfortable governing that If the purpose of an effect was legal when the effect was activated, then the effect continues even if the target becomes illegal later..
In summary, this DM, like yours, would also say that the family continues to benefit from the shared spell, even if the family member no longer has the ability to share spells after receiving the shared spell.