I would guess that Pr. Andrew is not saying or doing anything about things like this BECAUSE Sarah is attending the parties with them.
Something along the lines of : Daddy, can I please go this party? Mum will go with me!
In either case, it's pretty sad. I would not allow my daughter to go to an adult party with or with out me before the age of 18 and then only because she is 18 and considered a legal adult would I let her go with conditions. (isn't Eugenie 15 or 16?)
It's a catch 22 so to speak about what and how Sarah is going about this.
It's good that she is exposing her daughters to these types of parties and "probably" keeping an eye on them and keeping them from getting themselves in trouble.
But then on the other hand, she is exposing them way to early to these things. And letting them indulge in these type of activities now, can very easily lead these girls astray.
In either case, I feel sorry for these girls. They are trying to be normal teenagers, living a normal life, in a non-normal world, with non-normal parents.