How to trigger validation rules between child objects.

What would you do in this situation?

Parent A

 Child B

  Property B1

 Child C

  Property C2

Basically, there is a parent business object, with two child business objects, each with a property.

Now, what I need is to have a validation rule on Child B Property B1, that checks if the value in Child C Property C2 is the same.  

So this would need to be triggered whenever either Child B Property B1 changes, or Child C Property C2 changes.

I don't want Child B to have to know about the existence of Child C, or vice-versa.

This is a cut down example of a real-world issue, which is far more complex, but finding a simple approach to this should apply for the more complex.

BTW, it's impractical for me to have a replica of Child B Property B1 and/or Child C Property C2 on Parent A.

And I am quite ok to use multiple rules, and to trigger them from OnChildChanged, if that makes it easier.

I'm using CSLA 4.3.

I'm open to any suggestions.